What is the citation in MLA style for this story?

The citation for Jack London's story "To Build a Fire" in MLA (Modern Language Association) style (8th edition) depends upon how you accessed the work. If you accessed it through an anthology or a textbook that contains multiple stories, you would cite the anthology or textbook as the "container" of the work. However, if you accessed the work online, then you would cite the website as the "container" and give the URL and date accessed.
One anthology that contains London's story is the Norton Anthology of American Literature. The Works Cited citation for the story found in that book is as follows:
London, Jack. "To Build a Fire." Norton Anthology of American Literature, edited by Nina Baym, 8th ed., vol. E, W. W. Norton, 2012, pp. 1047 - 1058.
The in-text parenthetical citation would be: (London 1047) or whatever specific page you are quoting or paraphrasing from.
You can access the story online in many places since its copyright protection has expired. One site that features the entire story is the Library of America's Story of the Week. Here is how that works cited citation would look:
London, Jack. "To Build a Fire." Story of the Week, Literary Classics of the United States Inc., 25 Feb. 2011, http://storyoftheweek.loa.org/2011/02/to-build-fire.html. Accessed 8 Feb. 2018.
You would use the day that you look it up online as the "accessed" date. Since the website does not have page numbers, your in-text parenthetical would not need a page number. It would just need the author's name (like this): (London).
You must adapt your citation to reflect the actual source that you used to read the story. The above examples show the difference between a print and online citation in the MLA Style 8th edition.
https://style.mla.org/works-cited-a-quick-guide-book/