Parts of the following sentence have been given as options. Select the option that contains a grammatical error.
We were grateful for a book they gave us.

The indefinite article "a" in the phrase "a book" is incorrect because the book is specified by the adjective clause "they gave us". Since it refers to a particular, definite book, the definite article "the" must be used instead of "a". The correct sentence is: "We were grateful for the book they gave us."