The correct option is C Populous
Let’s examine the sentence: “Uttar Pradesh is the most ________ state of India.” The sentence requires an adjective for “state,” referring to Uttar Pradesh in this case. The two given are “popularity” and “populous.” The word “popular” is a noun and refers to “the fact of being popular.” The word “populous” is an adjective and means “having huge population.” Of the two adjectives, “populous” is suitable for “state,” for “populous state” means “a state with huge population.” Hence, Option B is the correct answer.