Final answer:
Voice was converted into fluctuating voltage in early telephones by turning sound waves into electrical signals through the movement of a diaphragm and a magnetic element within a coil to induce a current.
Step-by-step explanation:
The early telephones converted voice into a fluctuating voltage by converting sound waves into electrical signals (B). This conversion was achieved through a process where sound waves, which are mechanical vibrations in the air, were directed onto a diaphragm in the telephone's mouthpiece. This diaphragm then vibrated in response to the sound waves, and these vibrations were mechanically linked to a movable magnetic element situated within the coil of wire. As the magnetic element moved within the magnetic field of the coil, it induced an electrical current that fluctuated in accordance with the vibrations of the diaphragm. These electrical signals could then be transmitted over wires to a receiving telephone, where the process was reversed to recreate the original sound from the fluctuating electrical current.