Das has posted the letter.

The correct answer is: A. The letter has been posted by Das.

The other options are incorrect because they do not agree with the tense of the sentence “Das has posted the letter.”

• Option B, “The letter was posted by Das,” is in the past tense. However, the sentence “Das has posted the letter” is in the present perfect tense.
• Option C, “The letter had been posted by Das,” is in the past perfect tense. However, the sentence “Das has posted the letter” does not require a past perfect tense.
• Option D, “The letter have been posted by Das,” is in the plural form. However, the subject of the sentence is “Das,” which is singular.

Therefore, the correct answer is: A. The letter has been posted by Das.