Challenger

AKannan said that he had written a letter

BKannan said that he has written a letter

CKannan said that he had been writing a letter

DKannan said that he wrote a letter

Explanation:

When transforming direct speech into reported speech in the past, you change the tense of the verb from the past simple ("wrote") to the past perfect ("had written") to convey that the action took place before the reporting verb ("said").