what is the adjective clause in the following sentence and name the pronoun or noun it modifies "The book that you mentioned is no longer in print."

The relative/adjective clause is "that you mentioned".

Explanation:

An adjective clause is a dependent clause that acts as an adjective, modifying  or describing a noun in a sentence. It is also known as a relative clause.

In the given sentence "the book that you mentioned is no longer in print", the dependent clause is "that you mentioned". The sentence can also stand as a complete sentence without the dependent clause:

"The book is no longer in print."

But with the dependent clause, we are given more details about the book, describing the noun "the book".

Thus, the relative/adjective clause is "that you mentioned".