The correct answer is C. “She wrote a letter.”
The other options are incorrect because they do not accurately reflect the tense of the sentence. Option A, “A letter is being written by her,” is in the present progressive tense, which indicates that the action is happening now. Option B, “A letter is written by her,” is in the simple past tense, which indicates that the action happened at a specific time in the past. Option D, “A letter was being written by her,” is in the past progressive tense, which indicates that the action was happening at a specific time in the past but is no longer happening.
The sentence “She wrote a letter” is in the simple past tense because it indicates that the action of writing the letter happened at a specific time in the past. The action of writing the letter is complete, and it is not happening now.