Which of the following are the two main types of modulation used in radio transmission?

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The two main types of modulation used in radio transmission are indeed Amplitude Modulation (AM) and Frequency Modulation (FM). These techniques serve as fundamental methods for varying a carrier wave to encode information.

Amplitude Modulation (AM) changes the amplitude (strength) of the carrier wave in accordance with the audio signal, which is suitable for voice transmission over long distances. It allows the transmission of information by varying the signal's amplitude while keeping the frequency constant.

Frequency Modulation (FM) alters the frequency of the carrier wave in response to the input audio signal. This method is particularly effective in reducing static noise and provides better sound quality compared to AM, which is why FM is prevalent in music and high-fidelity sound applications.

While there are other forms of modulation listed in the other choices, such as Phase Modulation and various digital formats, they are either less common in conventional radio transmission or serve more specialized applications rather than being the primary methods of analog radio transmission. Thus, the choice of AM and FM captures the essence of the primary modulation techniques historically and practically used in radio broadcasting.