Choice ''d'' is correct. A downward sloping, fairly elastic (not inelastic) demand curve is a feature of a firm in monopolistic competition.Choices ''c'', ''a'', and ''b'' are incorrect as they are all features of monopolistic competition.Note: Firms in a monopolistically competitive market produce "similar" goods; however, unlike in a perfectly competitive market, the goods are NOT identical or perfect substitutes. The definition of being able to "influence prices" is that the firm has a downward sloping demand curve. A firm in a monopolistic competition environment has little market control, but it is still able to influence prices. It cannot influence them as much as a pure monopoly can, but the influence is still there. The firm is still not a price-taker; it is a price-maker (hence the downward-sloping demand curve), so, again, it is able to influence to some degree.