Answer:
A. The author often compares the sound of the cello to a human voice.
Explanation:
Medium is the channel or system through which communication is made. It is the process through which information is given out or processed out.
This process allows the deliverance of the main story or plot in a narrative or story. It is also characterized by the sort of work it is, like for example, a novel is one medium, a drama another. As such, the author comparing the sound of a cello to a human voice is one such medium. Thus, the correct answer is option A.