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

The correct option containing an error is a book.

Explanation:
In English grammar, when a noun is followed by a restrictive relative clause that identifies or specifies a particular object, we must use the definite article "the" rather than the indefinite article "a".

In the given sentence, "We were grateful for a book they gave us," the relative clause "they gave us" specifically identifies which book is being spoken about (a specific book, not just any random book). Therefore, the indefinite article "a" is grammatically incorrect and should be replaced by the definite article "the".

Corrected Sentence:
"We were grateful for the book they gave us."

Thus, the part of the sentence that contains the error is a book.