The correct answer is D) The two substances must be of different temperature. For heat transfer to occur between two substances, there must be a temperature difference between them. Heat will always flow from the substance with a higher temperature to the substance with a lower temperature, until the temperatures equalize. This is a fundamental principle of thermodynamics. The other options are incorrect because they do not represent an essential condition for heat transfer. Differences in state of matter, specific gravity, or pressure do not necessarily enable heat transfer on their own. The key requirement is the temperature difference that drives the flow of thermal energy.