The equilibrium position of a firm is attained

The equilibrium position of a firm is attained when the firm's Marginal Cost (MC) is equal to its Marginal Revenue (MR). Marginal Cost is the additional cost of producing one more unit of a good or service, while Marginal Revenue is the additional revenue generated from selling one more unit of a good or service. When the Marginal Cost of producing an additional unit of a good or service is equal to the Marginal Revenue generated from selling that additional unit, the firm is operating at its optimal level of output and has reached its equilibrium position. Therefore, option C, "when MC = MR," is the correct answer.