The correct answer is A) The button must be depressed to speak and to listen. Sound-powered telephones operate on the principle of converting sound energy into electrical energy and vice versa. The button on the handset must be depressed in order to both transmit and receive signals, as it activates the internal circuitry that allows the user to speak and listen through the same device. This is a fundamental operating requirement for sound-powered telephone systems used on ships and other maritime applications. The other options are incorrect because: B) the button is not used to ring the remote station, C) the button must be depressed for both speaking and listening, not just listening, and D) the button must be depressed for both speaking and listening, not just speaking.