Select the most appropriate option to fill in the blank. This is an ______ book.

Concept

English forms two participial adjectives from a verb such as "interest": the -ing form describes a person, thing, or activity that causes a feeling, whereas the -ed form describes a person or other sentient experiencer of that feeling. In the common determiner + adjective + noun pattern, the word between "a/an" and the head noun describes that noun; whether a noun can instead serve as an attributive modifier depends on the particular noun combination.

Application

In "This is an ______ book," the head noun is "book." A book can cause a reader to feel interest, so the participial adjective must express the cause of the feeling: "interesting." The completed noun phrase is "an interesting book."

Cross-check

• "interested" is the -ed adjective form; it would fit only if the blank described a being who feels interest (e.g., "the interested reader"), but here the noun is "book" — the source of the feeling, not its experiencer.

• "interests" functions as a verb (or the plural noun "interests"), so it cannot occupy an adjective slot directly after the article "an."

• "interest" is the base noun or verb. English permits it in established combinations such as "interest rate," but the intended descriptive phrase for a book that arouses curiosity is "interesting book."

So the sentence reads correctly as: "This is an interesting book."