The sentence has been given in the Active/Passive voice. From the given options, choose the one which best expresses the given sentence in the Passive/Active voice.
An action in a sentence can be represented in two ways, namely active and passive voice.
The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.
The correct answer is C) A letter was being written by Mona to her father.
Option C is correct as the given sentence is in the active voice and to transform it into the passive voice the subject of the given sentence (Mona) comes in the place of the object(a letter) and vice versa. Also, as the given sentence is in the past continuous tense, 'being' is used.
Options A, B, and D are not correct as the sentence structure is wrong, the tense used is not the same as in the given sentence, and the given sentence is not in the simple past tense respectively.