Answer:
A) the firm should hire additional workers.
Explanation:
if the marginal production of the tenth worker is 5 units or output and the price of each unit is $4, the the workers total marginal product revenue (MPR) = 5 units x $4 per unit = $20
Since the cost of hiring that tenth worker is $15 (less than MPR), then the company should hire more additional workers until the MRP = labor cost