His endeavours proved fruitful.

Answer: 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