The correct answer is c$.$ price equals marginal cost $($p $=$ MC$).$

Under perfect competition, the efficient level of output is produced because, at equilibrium, the price of the good or service equals the marginal cost $($MC$)$ of production. This ensures that the resources are allocated in the most efficient way, where the value of what society gains from producing an additional unit $($marginal benefit$)$ equals the cost of producing that unit $($marginal cost$).$ This condition leads to an efficient allocation of resources in the market.

a$.$ is incorrect because the government does not regulate the output in perfect competition.

b$.$ is incorrect because earning only a normal profit does not directly determine efficiency.

d$.$ is incorrect because firms cannot earn economic profits in the long run in a perfectly competitive market due to the entry of new firms.