Rewrite the sentence in active voice.
This letter was written by Mr. Kumar.

Correct option is D

Explanation: "This letter was written by Mr. Kumar" in passive voice translates directly to "Mr. Kumar wrote this letter" in active voice. This maintains the simple past tense, indicating a completed action.
Past indefinite
Subject + V2 + object. -> <- Object + auxiliary verb (was, were) + past participle + by + subject.
Example: The meal was prepared by the chef. -> The chef prepared the meal.