Passive voice is when the object is being acted upon by the subject, instead of the subject acting upon the object. Note: The subject of a sentence does the action, while the object receives the action. Look at the following sentences.
I ate the apple. -- This sentence is in active voice because the subject (me) acted upon (ate) the object (the apple). It follows the pattern subject-verb-object.
The apple was eaten by me. -- This sentence is in passive voice because the object (the apple) was acted upon (was eaten) by the subject (me). It follows the pattern object-verb-subject.
The sentence in passive voice is sentence D - Many artists were influenced by Elvis Presley's music.
If you were to rewrite this sentence in active voice, it would read "Elvis Presley's music influenced many artists." This follows the subject-verb-object pattern of active voice.