Question

as the number of firms in an oligopoly increases,
d. the output effect decreases.
the total quantity of output produced by firms in the market gets closer to the socially efficient quantity.
each seller becomes more concerned about its impact on the market price.
a. the oligopoly has more market power and firms earn a greater profit.

Explanation:

1. Analyze option d: The output effect in oligopoly—when a firm increases output, it raises its own revenue (output effect) and lowers price (price effect). As the number of firms increases, the price effect becomes more significant relative to the output effect, but the output effect itself doesn't necessarily decrease. So this option is incorrect.
2. Analyze the option "the total quantity of output produced by firms in the market gets closer to the socially efficient quantity": In an oligopoly, firms restrict output to increase profits (similar to a monopoly but with multiple firms). As the number of firms in the oligopoly increases, the market becomes more competitive (closer to monopolistic competition or even perfect competition in the limit). In perfect competition, the output is at the socially efficient level (where price equals marginal cost). So as the number of oligopoly firms increases, the total output increases and moves closer to the socially efficient quantity. This option is correct.
3. Analyze the option "each seller becomes more concerned about its impact on the market price": As the number of firms in an oligopoly increases, each firm's share of the market decreases. So each firm has less impact on the market price, and thus becomes less concerned about its individual impact on the market price. This option is incorrect.
4. Analyze option a: Market power in an oligopoly comes from the ability to restrict output and influence price. As the number of firms increases, competition increases, so each firm has less market power, and profits tend to decrease (due to more competition and lower prices). So this option is incorrect.

Answer:

the total quantity of output produced by firms in the market gets closer to the socially efficient quantity