6604. His endeavours proved fruitful.

✅ The correct answer is option D.
Justified : having, done for, or marked by a good or legitimate reason.

Futile : incapable of producing any useful result; pointless.

Efficient : (of a system or machine) achieving maximum productivity with minimum wasted effort or expense.

Productive : producing or able to produce large amounts of goods, crops, or other commodities.

Fruitful : producing good or helpful results; productive.

Synonym of Fruitful is Productive