In the microphone, used in the public address system.
(A) Electric signals are first converted into sound waves
(B) Sound waves are directly transmitted
(C) Sound waves are converted into electric signals which are amplified and transmitted
(D) Amplification is not required

Hint: Microphones are types of transducers. A public address system would require loud volume and clear transmission.

Complete Answer:
A public address system is for amplifying the human voice. In its simplest form, it has a microphone, mixer, and loudspeakers. It all starts with the microphone (mic), which converts sound pressure to voltage. That means when we speak or sing into the mic, its magnetic force outputs a small amount of voltage. That voltage is then sent to either a mixer or loudspeaker for amplification. Once boosted by a power amplifier, the voltage is so high that it forces the speakers to move and recreate the sound pressure changes which first entered the mic. The result is a much louder sounding voice.
Thus, in a public address system the sound waves are converted into electric signals, which are then amplified and transmitted. In other words, microphones are a type of transducer - a device which converts energy from one form to another.
Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal).
Hence, the correct answer is Option C.

Note:
Amplification is how the signal level, or its voltage level, is increased at separate gain stages of the system. Gain staging refers to each level of pre-amplification or volume adjustment found in a signal path. For a PA system, the gain stages are the microphone preamplifier, channel level, mix level, and loudspeaker level.
The voltage starts at mic level and, after being increased by a preamplifier gain knob, is increased closer to line level. Once raised to line level, the signal level is adjusted by a mixer’s level controls. When the mix leaves the mixer and enters the loudspeaker’s amplifier, it is dramatically increased to speaker level and controlled by the amplifier’s level control. The high voltage level causes the speaker to rapidly move back and forth, thus reproducing an amplified version of your original signal.