Answer: D. compete rather than cooperate with each other
Explanation: Oligopoly is a market form in which a particular market or industry is dominated by a few numbers of large sellers. Social optimum or socially optimal output is achieved when output occurs at the intersection of marginal social benefit and marginal social cost.
In order to achieve this, firms in oligopoly should be encouraged to compete rather than cooperate against each other so as to keep the price competitive and move the allocation of resources closer to the social optimum.