This book is a useful . . . . . . . to our library.

The correct answer is D. addition.

An addition is something that is added to something else. In this case, the book is being added to the library. The other options are not correct because:

• An arrival is someone or something that comes to a place.
• A discovery is something that is found or learned for the first time.
• An asset is something that is valuable or useful.

The book is a useful addition to the library because it will provide information and entertainment to the library’s patrons.