In perfect competition, many small sellers (a-1) ensure no individual influences price. Many small buyers (b-2) ensure no buyer influences price. Undifferentiated products (c-3) mean decisions are based on price. Easy entry/exit (d-4) ensures resource mobility.

In monopolistic competition, the absence of significant barriers to entry allows new firms to enter the market when existing firms are making economic profits. This entry increases the supply of similar products, which shifts the demand curve for individual firms to the left until economic profits are eliminated, resulting in zero long-run economic profit.

An oligopoly is characterized by a small number of firms, interdependence, and often price leadership. A horizontal demand curve indicates perfect elasticity, which is a feature of perfect competition, not oligopoly. In an oligopoly, firms face a downward-sloping demand curve, often with a 'kink' due to the strategic reactions of competitors to price changes.

The Prisoner's Dilemma is a classic example in game theory that illustrates why two completely rational individuals might not cooperate, even if it appears that it is in their best interests to do so. In the context of oligopoly, it explains why firms may struggle to maintain collusive agreements, as each firm has an incentive to cheat to maximize its own profit.

The correct pairings are: Indifference curves are convex to the origin (a-4), demand curves slope downward to the right (b-1), perfect competition is characterized by P=AR=MR (c-2), and price leadership is a common feature of oligopoly market structures (d-3).

In perfect competition, the firm is a price taker, meaning Price equals Marginal Revenue (P=MR). For long-run equilibrium, the firm must produce where Price equals Marginal Cost (P=MC). Since P=MR, the condition is satisfied when Price equals Marginal Revenue, ensuring no incentive for the firm to change its output level.

A monopolist does not have a supply curve because it is a price maker, not a price taker. The quantity supplied is determined by the intersection of marginal revenue and marginal cost, rather than being a function of price as in perfect competition.

In perfect competition, firms are price takers, meaning the price is constant regardless of the quantity sold. Since price equals Average Revenue (AR) and the additional revenue from selling one more unit (Marginal Revenue, MR) is also equal to that constant price, AR must equal MR at all levels of output, including the long run.

Monopolistic competition involves many sellers offering differentiated products. Because these products are close substitutes, the cross-elasticity of demand is high. While 'infinite' is technically associated with perfect competition, in many academic contexts, it is used to describe the high sensitivity of demand for one firm's product to the price changes of competitors in a monopolistically competitive environment.

In a perfectly competitive market, the firm is a price taker, meaning the price remains constant regardless of the quantity sold. Since Total Revenue (TR) = Price * Quantity, and Price is constant, the TR curve is a linear, upward-sloping straight line starting from the origin. Marginal Revenue is equal to the price, not greater than it.