The correct answer is: C. A letter was being written by Mona to her father.
The other options are incorrect because they do not accurately describe the action that is taking place in the sentence. Option A, “A letter was written to her father by Mona,” implies that the letter has already been written. Option B, “A letter has been written to her father by Mona,” also implies that the letter has already been written. Option D, “A letter was written by Mona to her father,” implies that the letter has been written and is now complete. However, the sentence states that Mona was writing the letter, which means that the letter is not yet complete.
The sentence is in the past progressive tense, which is used to describe an action that was in progress at a specific time in the past. In this case, the action is Mona writing the letter. The past progressive tense is formed with the past tense of the verb “to be” (was or were) and the present participle of the main verb (writing).
The sentence can be paraphrased as follows: “At some point in the past, Mona was in the process of writing a letter to her father.”