The preposition "to" is used when indicating the recipient of an action.
In this case, the letter is being written to the father.
The correct sentence should be:
"He wrote a letter to his father."
Thus, the correct answer is Option D ("to").
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?