Answer:
D some firms leave the industry and the existing firms slowly adjust their production to reach their minimum efficient scale.
Step-by-step explanation:
In a perfectly competitive industry at starting there is a short-run equilibrium in which all the firm is earning zero economic profit but these firm operated below the minimum efficient scale or we can say minimum requirement i.e lowering the average cost for the long run
By going through the options the option is correct as few firms leave the industry and other existing firms try to adjust the production in a slowly way so that they could reach their minimum efficient scale
Hence, the option d is correct