Concept:Audio aids use the sense of hearing to enhance learning. They play or reproduce sound.Explanation:Gramophone is a device that plays recorded sound. So it is an audio aid.Other examples of audio aids are radio, tape recorder, and linguaphone.The remaining options – O.H.P., Epidiascope, and Magic Lantern – are projective aids. They use both sight and sound to show images on a screen.Thus, only Gramophone works purely through hearing.Answer:C. Gramophone