You have to understand positive, comparative, and superlative forms of adjectives to do this. The positive form is the simple form of an adjective as applied to the sentence. The comparative form refers to the form of an adjective that relates it to other possible intensities of the adjective; it it used to say to what extent an adjective applies. The superlative form states the adjective in the most prominent extent. Alright, now let's look at the answers.
Answer A is incorrect because it should be "cheap, cheaper, and cheapest."
Answer B is incorrect because it should be "drowsy, drowsier, and drowsiest."
Answer C is correct.
Answer D is incorrect because is should be "troublesome, more troublesome, and most troublesome."
