English Vocabulary — Choose the closest meaning (contextual synonym). Sentence: The time I spent in the library was a most rewarding one.

Introduction / Context:
“Rewarding” describes activities that yield benefit, satisfaction, or positive returns. In academic and professional settings, it often refers to intellectual or personal gain rather than money. This question asks for the nearest single-word substitute that keeps the sentence natural and idiomatic.

Given Data / Assumptions:

• Focus adjective: rewarding.
• Context: time spent in a library (learning, knowledge gains).
• Options: profitable, paying, serviceable, precious.

Concept / Approach:
Among the choices, “profitable” best captures the idea of yielding benefit or advantage, even beyond finance, as in “a profitable discussion.” “Paying” primarily relates to financial return and feels odd here. “Serviceable” means usable but not especially beneficial. “Precious” means valuable in an emotional sense, which shifts the nuance from benefit to sentiment. Therefore, “profitable” is the most suitable synonym in common exam usage.

Step-by-Step Solution:

Verification / Alternative check:
Many dictionaries list figurative “profitable” = advantageous or useful, not solely monetary. In educational contexts, this is standard and idiomatic.

Why Other Options Are Wrong:

• paying: Narrowly monetary; awkward for time spent learning.
• serviceable: Merely adequate/usable; lacks benefit emphasis.
• precious: Emotional value; not the same as productive benefit.

Common Pitfalls:
Assuming “profitable” must be financial. In formal English, it often extends to non-monetary gains such as knowledge or insight.

Final Answer:
profitable