<span>The correct answer is D. The book that explains the plays is on the shelf. In this sentence, "book" is the noun, which is singular. This means that the verb "to be" should also be singular, resulting in "is." A is incorrect because the verb "to be" should match the noun "reasons," which is plural. B is incorrect because the verb "to be" should match "people," which is plural. C is incorrect because, again, the verb "to be" should match the plural noun "reasons." </span>
