The time I spent in the library was the most rewarding one.

Answer: Option C

Precious : of great value; not to be wasted or treated carelessly.
Serviceable : fulfilling its function adequately; usable.
Profitable : beneficial; useful.
Paying : give (someone) money that is due for work done, goods received, or a debt incurred.
Rewarding : providing satisfaction; gratifying.

Synonym of Rewarding is Profitable