7216. The time I spent in the library was a most rewarding one.

✅ The correct answer is option A.
profitable : (of a business or activity) yielding profit or financial gain.

paying : give (someone) money that is due for work done, goods received, or a debt incurred.

serviceable : fulfilling its function adequately; usable.

precious : of great value; not to be wasted or treated carelessly.

rewarding : advantageous; productive.

Synonym of rewarding is profitable