Which one of the following statements best defines the concept of heat ?

Heat is fundamentally defined in thermodynamics as the form of energy that is in transit or transferred between systems or a system and its surroundings solely due to a temperature difference. It flows spontaneously from a body at a higher temperature to a body at a lower temperature until thermal equilibrium is reached.

• Option A: Describes the general principle of energy transformation (e.g., mechanical to electrical), which is not specific to heat.
• Option B: Relates to mass-energy equivalence ($E=mc^2$), which is a concept in relativity and not the definition of heat.
• Option D: Describes thermal expansion, which is a physical effect or property change resulting from temperature variations, rather than the definition of heat itself.

Thus, Option C is the correct definition as it identifies heat as energy transfer driven by thermal gradients.