The correct answer is C) When heat is transferred from one body to another, the direction of heat flow is unidirectional from the relatively hot body to the relatively cold body. This statement represents the second law of thermodynamics, which states that heat naturally flows from a hotter object to a colder object and not the other way around. The direction of heat flow is always from the higher-temperature body to the lower-temperature body, and this process is unidirectional. The other options are incorrect because: A) This statement describes the correct direction of heat flow but uses the term "relatively cold body" instead of "relatively hot body." B) This statement suggests the direction of heat flow is unpredictable, which is contrary to the second law of thermodynamics. D) This statement describes a bidirectional heat flow, which is not consistent with the second law of thermodynamics.