Firms gain control over price in monopolistic competition by a) blocking entry of other firms into the industry. b) producing a product for which there are no close substitutes. c) differentiating their products. d) colluding with other firms to set prices.

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.