Our talks with the foreign delegation were really productive.

Answer: Option C

Growing : (of a living thing) undergoing natural development by increasing in size and changing physically.
Efficient : (of a system or machine) achieving maximum productivity with minimum wasted effort or expense.
Fruitful : producing good or helpful results; productive.
Lengthy : (especially in reference to time) of considerable or unusual length, especially so as to be tedious.
Productive : producing or able to produce large amounts of goods, crops, or other commodities.

Synonym of Productive is Fruitful