Long-run costs and economies of scale — practice question

At what stage does a profit-maximising firm employ the optimum number of workers?

• Awhen the average revenue product of labour equals the average cost of hiring workers
• Bwhen the marginal revenue product of labour equals the average cost of hiring workers
• Cwhen the marginal revenue product of labour equals the marginal cost of hiring workers
• Dwhen the marginal revenue product of labour equals the trade union supplied cost of workers