The answer to your question would be that the sentence that is written in the active voice is the following one: Samantha returned the book herself. That is, the correct option would be A.
In most English sentences with an action verb, the subject performs the action denoted by the verb. Since in these sentences the subjects does or acts upon the verb, the sentences are said to be in the active voice. As you can see, in sentence A Samantha, the NP which is the subject of the sentence, is the one who returns the book.
