A. A letter was written to her father by Mona.
B. A letter has been written to her father by Mona.
C. A letter was being written by Mona to her father.
D. A letter was written by Mona to her father.
Given sentence is in past continuous tense and it is in active voice.
Rule :
Subject + (was /were) + being + V3 + Optional Agents.