Select the option that expresses the given sentence in passive voice.

The postman had delayed the letter.

The letter had been delayed by the postman.

The correct passive form of the sentence The postman had delayed the letter is The letter had been delayed by the postman.

The active voice of the sentence is The postman had delayed the letter. This sentence is in the past perfect tense, which is used to describe an action that happened in the past before another action happened. 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 "letter" and the main verb is "delay." The past participle of "delay" is "delayed." So, the passive form of the sentence is The letter had been delayed by the postman.

The other options are incorrect because:

• The letter had delayed the postman. is in the active voice.
• The letter was delayed by the postman. is in the simple past tense, but the past perfect tense is more appropriate because the action of the letter being delayed happened before another action happened (the postman leaving).
• The postman had been delayed by the letter. is in the past perfect tense, but the subject of the sentence is incorrect. The letter is not the one who delayed the postman.