The correct answer is C. Cuesta; 900
Step-by-step explanation:
In Spanish, the verb form varies depending on the subject that precedes the verb and tense (present, past, future). In the case of the verb "costar" that is the verb of the sentence and means cost this verb is mainly used to describe the price of one or more products. In this sense, you use "cuesta" if there is only one object or "cuestan" if there are more objects. According to this, in the case of the sentence "El sofá ________ novecientos dólares" the correct verb is "cuesta" because "El sofá" is a singular object that refers to one couch, this can be identified because the noun does not end in -s. Besides this, in Spanish the number "900" is known as novecientos, this word derives from "nueve" (9) and "cien" (100). Thus, the correct verb is "cuesta" and the correct price is "900".