Final answer:
The profit-maximizing condition for a Cournot oligopolist is when marginal revenue equals marginal cost, which is option a. MR = MC.
Step-by-step explanation:
The profit-maximizing condition for a Cournot oligopolist, similar to that of a perfectly competitive firm or a monopoly, is when marginal revenue (MR) is equal to marginal cost (MC). Therefore, the correct answer is a. MR = MC. This rule applies to various market structures and suggests that firms will maximize profits by producing a quantity at which the additional revenue from selling one more unit equals the additional cost of producing that unit. It's important to recognize that while b. Q1 = Q2 = ... = Q could imply some level of collusive behavior like a cartel, and c. P = MR may occur under perfect competition, neither of these conditions is inherently about profit maximization for a Cournot oligopolist, which focuses on MR = MC.