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").
Each question has a sentence that has an underlined part. Choose the option which rephrases the underlined part in the most appropriate manner in terms of grammar and usage.
In an attempt to boost their profits, many edible oil-producing companies have been engaging themselves in propaganda against commonly used oils and promoting exotic and expensive varieties of oils \(\underline{as\ more\ healthier\ options.} \)