Question

what do the terms am and fm refer to?
high and low frequencies of sound
the way sound moves along a cable network
how an electromagnetic carrier wave is altered
the number of radio channels available

Explanation:

AM (Amplitude - Modulation) and FM (Frequency - Modulation) are methods of altering an electromagnetic carrier wave to transmit information. In AM, the amplitude of the carrier wave is varied, while in FM, the frequency of the carrier wave is varied.

Answer:

C. how an electromagnetic carrier wave is altered