Identify the form of energy that is converted into thermal energy by your body.

Different names are sometimes used for the different forms of energy, so I’m going to include some explanation to make sure there’s no confusion.
Thermal energy is the energy of motion of the atoms and molecules that make up a substance or a living organism. Temperature is related to thermal energy and is proportional to the average kinetic energy (energy of motion) of the atoms and molecules. One of the reasons our bodies need energy is to convert it to thermal energy, so as to maintain a constant body temperature. This is part of homeostasis, maintaining constant conditions so that the processes of life can function consistently.
We obtain energy one way: by eating food. Energy in food is stored in the chemical bonds within molecules of the foodstuff. The energy stored in food is released in a process that breaks higher-energy chemical bonds and replaces them with lower-energy bonds. The difference in energy is often called chemical potential energy.
Potential energy is a general term for energy that is, in principle, available for conversion to other forms. Gravitational potential energy is the energy stored in an object at some height above the ground, while latent heat, or phase potential energy, is potential energy that a substance has by virtue of the way its atoms or molecules are arranged as a solid, liquid, or gas. Similarly, chemical energy, or chemical potential energy, is the energy that is available when bonds are broken and new ones are formed.
Since all energy used by our cells is derived from breaking and reforming bonds in molecules which originate in our food, the source of thermal energy in our bodies must be chemical potential energy.