What is defined as the thermal energy that flows from one substance to another when there is a temperature difference?

The correct choice, which is heat, is specifically referred to as the thermal energy transfer that occurs between substances due to a temperature difference. When two objects are at different temperatures, heat flows from the hotter object to the cooler one until thermal equilibrium is reached. This transfer is a fundamental process in thermodynamics and is essential for various physical phenomena, such as warming your hands when you hold a cup of hot coffee.

In contrast, energy is a broader concept that encompasses various forms, such as kinetic, potential, thermal, and more, without being specific to thermal transfer. Temperature refers to the measure of the average kinetic energy of particles in a substance, but it does not directly define the transfer of energy between substances. Thermal conductivity, on the other hand, is a material property that indicates how well a material can conduct heat, but it is not a definition of the energy transfer process itself. Thus, heat is the only term here that accurately describes the phenomenon of thermal energy moving from one substance to another due to a temperature difference.