What were the impacts of the ideas of Darwin, Marx, Freud, and Einstein upon popular culture and the mass society of the early twentieth century? Give specific examples. Which of these seminal intellectuals remains most relevant in the early twenty-first century, and which of the four seems to have the least relevance, and why?

Darwin, Marx, Freud, and Einstein are some of the most consequential thinkers in modern society. Their influence is felt far beyond their respective fields.
Charles Darwin is best known for his contribution to the theory of evolution. He theorized that all life, including humans, descends from common ancestors, and that changes in heritable characteristics are influenced by natural selection. Heritable traits survive or die within biological populations based on whether they contribute to the survival of the species. Darwin’s theory denies the anthropocentric views that underlie many religious doctrines dedicated to pre-scientific origin stories.
Karl Marx is probably the thinker whose ideas can most directly be linked to widespread social, political, and cultural transformations. While most scholars and even many of Marx’s fiercest critics would concede that Soviet Marxism deviated greatly from the democratic impulse at the core of Marx’s writings, his profound influence on Lenin’s Bolshevism is undeniable. Nonetheless Marx’s work, which stresses the dominant role of economic rationality in how society is organized, continues to inspire and animate contemporary discussions about the economy, especially in the wake of the 2007–2008 financial crisis.
Sigmund Freud developed the theory of psychoanalysis, which is a therapeutic mechanism that seeks to solve the riddle of the human mind through the unconscious. Freud’s ideas were adopted by his nephew Edward Bernays, who is widely considered “the father of public relations.” Bernays often said that he came up with the term "public relations" as an alternative to the highly stigmatized word “propaganda.” His hope was to employ the central tenets of Freudian psychology to help businesses sell products and for governments and politicians to sell ideas. Advertisements no longer focused solely on the utility of a particular product, but sought to sell a feeling—a way of life.
Albert Einstein’s breakthrough theory of relativity remains one of the backbones of modern physics. While Isaac Newton before him came up with a formula on the strength of gravity, Einstein’s theory discovered the origin and source of gravitational pull. Einstein’s work forms a fundamental basis for physicists asking big questions about the laws of the universe.
As for which one of these intellectuals remains most relevant or the least, that is a matter for debate. Each of these thinkers still dominates their respective fields and their ideas live on and contribute to popular ideas about the nature of reality.
https://plato.stanford.edu/entries/darwinism/

https://plato.stanford.edu/entries/marx/

Charles Darwin, Karl Marx, Sigmund Freud, and Albert Einstein each have contributed greatly to the twentieth century up to the present. The commonality between these four great thinkers is that their contributions were theoretical.
Charles Darwin theorized that living things on Earth evolved through natural selection. Karl Marx questioned the economic, social and political hierarchies of past feudal states and eventually wrote "Das Kapital" and "The Communist Manifesto," two seminal works that brought forth a new way of seeing social and economic dynamics. Sigmund Freud revolutionized the then-new scientific branch of psychology, by introducing new psychiatric techniques. He concluded that human behavior is driven by primal, subconscious processes. Albert Einstein would pace around his room doing thought experiments until he connected the dots with the Theory of Relativity, which would lead to other breakthroughs in theoretical physics by future physicists.
The other common ground between these intellectuals is that their theories had a wide-ranging impact in real life. Darwin's theory of evolution sparked fierce debate in the United States during the mid-20th century, which would eventually pave the way for scientific topics, no matter how controversial, to be taught in schools. Marx's theories on socialism, capitalism and communism would, arguably, have the most impact on our civilization up to this day. Many political leaders interpreted Marx's theories into their own definitions, which has led to world-changing revolutions, civil wars, world wars and the Cold War. Some of the most powerful nations were directly influenced by Marx's theories, such as the former Soviet Union and modern-day China and North Korea.
Meanwhile, Freud's theories and practice of psychiatry is still used today, even if it is considered outdated by modern psychologists. However, his theories in psychology has also led to the birth of marketing and contributed to the advertising industry. His daughter, Anna Freud, would continue his work and infuse his theories into other industries. Albert Einstein's Theory of Relativity was not only groundbreaking work within the scientific community, it also led to the creation of the atomic and hydrogen bombs.
Of the four, Freud had the "least" impact, but only because Marx, Einstein and Darwin contributed more. However, Freud's indirect influence on modern-day advertising should not be discounted. Overall, these four individuals have shaped human civilization and will continue to do so in the future.

The work of Charles Darwin, Karl Marx, Sigmund Freud, and Albert Einstein demonstrates the power of thought and intellectual achievement to affect the broad trends in human culture and values. Each of these great thinkers contributed ideas that remain influential in Western society in the twenty-first century. From Darwin's work in the natural sciences have come the powerful notions of natural selection, evolution, and survival of the fittest. Marx's writings—leaving aside the political impacts of socialism and communism that flowed from his theories—revealed the influence of money (or capital) and wealth as a lens for viewing the relations and conflicts between social classes, and indeed all of history. Freud's radical approach to psychology illuminated the hidden operations of the mind and the ways unconscious motivations and repressed memories can drive human behavior. Einstein's insights, especially his famous theory of relativity, opened up rich new fields of scientific inquiry and helped confirm humanity's belief that the human mind is capable of understanding the principles determining how nature and the universe behave.
All these concepts were important parts of the transformation into "modernity" in the first half of the twentieth century. Like technological advances such as air travel, television, and electrical appliances, these ideas contributed to the sense of humanity's advance beyond the traditions and superstitions of the past.