The correct answer is A. precious.
The word “precious” means “of great value or importance.” In this context, the speaker is saying that the time they spent in the library was very valuable to them. They may have learned a lot, or they may have enjoyed the peace and quiet of the library. Whatever the reason, the speaker feels that the time they spent in the library was well worth it.
The other options are not as accurate. The word “serviceable” means “able to be used for a particular purpose.” In this context, it would not make sense to say that the time the speaker spent in the library was serviceable. The library is not a tool that can be used for a particular purpose.
The word “profitable” means “making or yielding a profit.” In this context, it would not make sense to say that the time the speaker spent in the library was profitable. The library is not a business that makes money.
The word “paying” means “giving or receiving money in return for goods or services.” In this context, it would not make sense to say that the time the speaker spent in the library was paying. The library is a free service.