The correct answer is: (A) and (R) both are true.
In the long run, all firms in a perfectly competitive market will earn only normal profit. This is because firms will enter the market if there are profits to be made, and will exit the market if there are losses to be made. As firms enter the market, the supply of the good will increase, which will drive down the price of the good. As the price of the good decreases, firms will experience a decrease in marginal revenue, which will cause them to decrease output. This process will continue until all firms are earning only normal profit.
In the long run, all firms in a perfectly competitive market will incur minimum average cost. This is because firms will have had time to adjust their plant size to the level that minimizes average cost. In addition, firms will not incur any selling costs, as there is no product differentiation in a perfectly competitive market.
Therefore, both assertion (A) and reason (R) are true.