Which of the following is a criticism of the theory of monopolistic competition?

Here is a more detailed explanation of each criticism:

• It is difficult to define a monopolistically competitive market and to determine the firms and products that comprise it. One of the main problems with the theory of monopolistic competition is that it is difficult to define a monopolistically competitive market. In order to determine whether a market is monopolistically competitive, we need to know how many firms there are in the market, how similar the products are, and how much market power each firm has. However, it is often difficult to obtain this information.
• When product differentiation is slight, each firm’s demand curve is nearly horizontal so the perfectly competitive solution provides an adequate approximation to the monopolistically competitive solution. Another problem with the theory of monopolistic competition is that when product differentiation is slight, each firm’s demand curve is nearly horizontal. This means that each firm has very little market power and can only charge a price that is very close to marginal cost. In this case, the perfectly competitive solution provides an adequate approximation to the monopolistically competitive solution.
• When there are strong brand preferences and few producers of many differentiated products, or when there are many producers but only a few compete as rivals for any given consumer, the oligopoly solution then provides an adequate approximation to the monopolistically competitive solution. Finally, when there are strong brand preferences and few producers of many differentiated products, or when there are many producers but only a few compete as rivals for any given consumer, the oligopoly solution then provides an adequate approximation to the monopolistically competitive solution. In this case, the firms in the market have a significant amount of market power and can charge a price that is above marginal cost.