The answer to the question above would be that the sentence that is written in the active voice is the following one: The science director's assistant placed the order yesterday afternoon.
This sentence is written in the active voice because it includes the doer of the action denoted by the verb. It indicates that the science director's assistant is the one who placed the order. He/she is the one who acts upon the action denoted by the verb. That's why it is called active voice. On the contrary, the other examples are in the passive voice because in those structures the doer of the action (or agent) has been supressed to emphasize the action and not the person doing it. Because the subject is being "acted upon" (or passive), the sentences are said to be in the passive voice.
