Select the correct passive form of the given sentence.

Who has taken away my pen?

By whom has my pen been taken away?

The correct passive form of the given sentence is By whom has my pen been taken away?

The active voice of the sentence is Who has taken away my pen? This sentence is in the simple present tense, which is used to describe an action that is happening now or that is happening repeatedly. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "who" and the main verb is "take away." The past participle of "take away" is "taken away." So, the passive form of the sentence is By whom has my pen been taken away?

The other options are incorrect because:

• By whom had my pen been taken away? is in the past perfect tense, while the original sentence is in the simple present tense.
• By whom is my pen taken away? is in the present progressive tense, while the original sentence does not imply that the taking away of the pen is an ongoing action.
• By whom was my pen taken away? is also in the past tense, but it does not use the past participle of the main verb.
• My pen has been taken away by whom? is not a complete sentence. It lacks a subject.