In the long run, each firm in a competitive industry earns
a. zero accounting profits.
b. zero economic profits.
c. positive, negative, or zero economic profits.
d. positive economic profits.

The correct answer is option B.

Explanation:

In a competitive industry there is no restriction on entry or exit of firms in the market. So, when in the short run the firms are enjoying super normal profits or positive economic profits, this would attract potential firms to join the industry in the long run.

As a result the industry supply will increase in the long run. The increase in supply would cause the price to fall. This would further contribute in reducing revenue and profit.

This process will continue till the profit is reduced to zero. If profit falls below zero, then firms incurring loss will exit the industry. Then again zero profits will be restored by reduction in supply and increase in price.

So, we can say that perfectly competitive firms will have zero economic profits or only normal profits in the long run.