Answer: Option(b) is correct.
Step-by-step explanation:
Correct option: Average total cost of the marginal firm.
In the long run, when new firms enter into market then the price of the product falls because of the competition and supply of output increases which also increases the cost of production.
So, firms in this market adjust untill the price equal to the minimum of average total cost. After this point it incurred losses, thus, firms left the market.