Final answer:
Perfect competition results in price being set equal to marginal cost, ensuring allocative and productive efficiencies. A monopoly sets prices above marginal cost, leading to lower quantity produced and higher prices than under perfect competition.
Step-by-step explanation:
Under perfect competition, the price is set equal to marginal cost, which reflects both allocative efficiency and productive efficiency. In contrast, in a monopoly, the firm maximizes profit by setting the price above the marginal cost, which leads to allocative inefficiency as the price exceeds the cost of production, causing a deadweight loss to society. Therefore, the quantity produced under monopoly is lower and the price charged is higher compared to a perfectly competitive firm.