In a perfectly competitive market, the process of entry and exit will end when (i) accounting profits are zero. (ii) economic profits are zero. (iii) price equals minimum marginal cost. (iv) price equals minimum average total cost.

 (ii) economic profits are zero

Explanation:

A perfect competition is characterised by many buyers and sellers of homogenous goods and services. Market prices are set by the forces of demand and supply. There are no barriers to entry or exit of firms into the industry.

In the long run, firms earn zero economic profit. If in the short run firms are earning economic profit, in the long run firms would enter into the industry. This would drive economic profit to zero.

Also, if in the short run, firms are earning economic loss, in the long run, firms would exit the industry until economic profit falls to zero.

I hope my answer helps you