The correct answer is A. The sound-powered transmitter and receiver units are located in component A, which is the sound-powered telephone handset. Sound-powered telephones rely on the conversion of sound energy into electrical energy and vice versa, without the need for an external power source. The handset contains both the transmitter (which converts sound into electrical signals) and the receiver (which converts electrical signals back into sound). The other options, B, C, and D, are likely other components of the sound-powered telephone circuit, such as the wiring, switchboard, or other accessories, but the key components of the transmitter and receiver are housed within the handset, which is option A.