What term describes the alteration of the amplitude or frequency of an electromagnetic wave in response to a second signal?

The term that describes the alteration of the amplitude or frequency of an electromagnetic wave in response to a second signal is modulation. Modulation is a fundamental concept in telecommunications and signal processing, where it involves varying a carrier signal's properties—such as its amplitude, frequency, or phase—in accordance with the information signal. This process allows for the transmission of data over various media, enabling effective communication.

In contrast, reflections pertain to the bouncing back of waves when they hit a barrier, without altering the wave’s original properties. Refractivations, while not a commonly used term in this context, would imply a change in the direction of waves, typically associated with the bending of light or other waves as they pass through different mediums. Amplifications refer to the increase in the strength of a signal but do not imply any changes in the signal's modulation characteristics. Hence, the correct choice is modulation, as it directly addresses the key aspect of altering the properties of the wave in relation to another signal.