Rewrite the sentence in passive voice:

The dog bit the boy.

Option A is the correct answer. The two kinds of grammatical voices are active and passive. When the subject is on the receiving end of action, the sentence is in passive voice. Active sentences with the pattern "has/have/had + past participle" will be rewritten into passive sentences with the pattern "has/have/had + been + past participle." Also, if the meaning is clear from the rest of the sentence, adding by+agent can be avoided.