Choose the correct statement for amplitude modulation

We need to identify the correct statement about amplitude modulation.

Amplitude modulation (AM) involves three signals. The information signal, also called the message signal, contains the actual information to be transmitted (e.g., audio). The carrier signal is a high-frequency signal used to carry the information over long distances. After the modulation process, the output is the modulated signal.

In AM, the amplitude of the carrier signal is varied in proportion to the instantaneous value of the information (message) signal, while the frequency and phase of the carrier remain unchanged.

To evaluate the given options, note that Option A states "Amplitude of modulating signal is varied…," which is incorrect because the modulating signal is the information signal itself and its amplitude is not varied. Option B asserts "Amplitude of modulated signal is varied in accordance with the information signal," but this describes a consequence rather than the defining process since the modulated signal is the result of modulation. Option C correctly states "Amplitude of carrier signal is varied in accordance with the information signal," which matches the definition of AM. Option D repeats the error of describing the modulated signal rather than the carrier.

Therefore, the correct answer is Option C: Amplitude of carrier signal is varied in accordance with the information signal.