How is temperature defined in terms of molecular activity?

Temperature is fundamentally defined as a measure of the average kinetic energy of the molecules in a substance. It is closely linked to how rapidly the molecules of a body are moving or vibrating. As temperature increases, the molecules gain more energy and move faster, which can be observed in their increased vibration.

The option that states temperature is "a measure of the speed at which a body's molecules vibrate" accurately captures this relationship, emphasizing how temperature reflects molecular activity. In essence, higher temperatures correlate to greater molecular motion, while lower temperatures indicate reduced molecular activity, making this definition pivotal in understanding thermal dynamics.

The other choices address different concepts related to heat and energy. The measurement of heat energy is not the same as temperature itself; instead, it refers to the quantity of thermal energy transferred between bodies. Heat transfer by electromagnetic waves pertains to one specific method of heat transfer (radiation), but it does not define temperature directly. Lastly, the concept of energy supplied to a body or substance relates more to the transfer of energy rather than the intrinsic measurement of kinetic activity, which temperature specifically deals with.