Heat is a form of energy that may be transferred due to a difference in what?

Heat is defined as a form of energy that is transferred between systems or objects with different temperatures. This transfer occurs because of the temperature difference; thermal energy naturally flows from a hotter object to a cooler one until thermal equilibrium is achieved. This principle is foundational in thermodynamics and explains many physical processes, such as the melting of ice or the warming of a room when a heater is turned on.

The other options, while they relate to physical properties of substances, do not directly influence the transfer of heat. Pressure affects the states of matter and can influence temperature under certain conditions, but it is not the principal factor in the transfer of heat energy. Volume can affect the density and, in turn, the specific heat capacity of a substance, but again, it does not serve as a direct cause for heat transfer. Humidity refers to the amount of water vapor in the air and affects thermal comfort and heat exchange rates, but it does not serve as the foundational principle for heat transfer. Therefore, temperature difference is the primary driving force in the transfer of heat, making it the correct answer.