Why does thermal energy occur?

Thermal energy is fundamentally linked to the motion of particles within matter. It arises from the kinetic energy associated with the random motions of atoms and molecules. In solids, liquids, and gases, these particles are in constant motion, colliding with one another and moving in various directions. As their speed increases, so does their kinetic energy, which then contributes to the overall thermal energy of the substance.

The relationship between motion and thermal energy explains why temperature serves as a measure of this energy; higher temperatures indicate greater average kinetic energy among particles. This understanding is essential in the study of thermodynamics and energy transfer, as it highlights the microscopic processes that underlie macroscopic thermal phenomena.

In contrast, while chemical reactions can involve heat and energy changes (as noted in option A), they are not the sole cause of thermal energy. Gravitational force, mentioned in option C, does not directly contribute to thermal energy but can affect the state of matter under certain conditions. Lastly, while heat sources can elevate the thermal energy of a system, it's the kinetic motion of particles that fundamentally constitutes thermal energy, making the explanation given in the correct choice the most accurate.