The correct answer is: D. a-2, b-4, c-1, d-3
Perfect competition is a market structure in which there are many buyers and sellers of a homogeneous product, and no one buyer or seller has a significant impact on the market price.
Monopolistic competition is a market structure in which there are many buyers and sellers of a differentiated product, and each seller has some degree of market power.
Monopoly is a market structure in which there is only one seller of a good or service.
Oligopoly is a market structure in which there are a few large sellers of a good or service.
In perfect competition, firms are price-takers, meaning that they cannot set the price of their product. The price is determined by the market, and firms must accept this price. This is because there are so many firms in the market that each firm has a very small share of the market. As a result, no one firm can have a significant impact on the market price.
In monopolistic competition, firms are price-setters, meaning that they can set the price of their product. However, firms in monopolistic competition face a downward-sloping demand curve, meaning that they must lower their price in order to sell more units. This is because firms in monopolistic competition produce a differentiated product, which means that there are some substitutes available for their product. As a result, firms in monopolistic competition have some degree of market power, but they do not have as much market power as a monopoly.
In monopoly, there is only one seller of a good or service. This means that the monopolist has a great deal of market power. The monopolist can set the price of its product without fear of losing customers to other firms. As a result, the monopolist can charge a higher price than firms in perfect competition or monopolistic competition.
In oligopoly, there are a few large sellers of a good or service. This means that the firms in an oligopoly have a significant impact on the market price. The firms in an oligopoly are interdependent, meaning that the actions of one firm can have a significant impact on the other firms in the market. As a result, firms in an oligopoly often engage in strategic behavior, such as price-fixing and collusion.