Which provides the main purpose of the underlined adjectival clause? the book that is on the black chair belongs to my english teacher. a) gives readers more information about the location of the book b) gives readers more information about the chair c) gives readers more information about the teacher d) gives readers more information about the speaker of the sentence

The answer is A: gives readers more information about the location of the book. This sentece provides the purpose of the underlined adjectival clause because the speaker is refering to the book that belongs to the teacher, and the clause that is on the black chair  specifies where the book is. Maybe there are other books that don't belong to the teacher, so the speaker has to inform the right place of that book.

A)gives readers more information about the location of the book

An adjectival clause describes a noun, and it often comes immediately after the noun it describes. The adjectival clause in this sentence is "that is on the black chair". It comes directly after the noun book. It describes the location of the book. The other options are wrong because the phrase describes the book not the chair, the teacher, or the speaker of the sentence.