Answer: Firms gain control over price in monopolistic competition by "C) differentiating their products.".
Explanation: Monopolistic competition is an imperfect type of competition in which there is a high number of sellers in the market. The products offered are characterized by having some differentiation and it is precisely this differentiation that makes these companies enjoy a certain power of market, have a certain voice when setting their prices and are not merely "price-acceptors", as in the case of perfect competition.