Answer: C) Taro has been appointed to the committee by the principal.
Explanation: passive voice is a form of a verb in which the subject receives or undergoes the action of the verb (instead of performing it, like it is in active voice). From the given options, the only one that has a verb in the passive voice, is the corresponding to option C: Taro has been appointed to the committee by the principal, the one that performs the action is the principal, but Taro (that receives the action) is the subject.