Final answer:
The only form of recorded music before the invention of the phonograph was sheet music, as radio broadcasts and gramophone records came after its invention. The correct answer is option (D)
Step-by-step explanation:
The single form of recorded music before the invention of the phonograph was sheet music. Before the phonograph, invented by Thomas Edison in 1877, there was no technology available to record and reproduce sounds. Music was either played live or it was distributed in the form of sheet music, which is a written representation of music that musicians read and perform from. Radio broadcasts and gramophone records only became possible after the phonograph's invention. Gramophone records are essentially a continuation of phonograph technology, and radio broadcasts did not begin until the early 20th century, well after Edison's patents. Thus, the correct answer to the question is 'a) Sheet music', as it was the only way to 'record' music for others to play back prior to the technological advancements of the phonograph.