A Nash equilibrium occurs when each participant in a strategic interaction chooses the best available strategy given the strategies chosen by others. Option C is correct because no actor has an incentive to change its strategy unilaterally once the equilibrium is reached. This concept is central to game theory and is especially useful in oligopoly analysis, where firms must consider how rivals will respond to pricing, output, advertising, or product decisions. Option A describes the prisoner's dilemma more specifically, which can produce a Nash equilibrium but is not the definition itself. Option B describes collusion or cartel behavior. Option D describes illegal coordinated action by firms. Managers use Nash equilibrium logic to anticipate competitor behavior and understand why mutually beneficial cooperation can be unstable.

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