Concept:Optimum size of a firm is the scale of production where it produces efficiently at minimum cost.Explanation:At optimum size, the firm attains the lowest possible average cost for the greatest possible output.This is the point of productive efficiency on the long-run average cost curve.Marginal cost equals marginal revenue is a profit-maximising condition, not the definition of optimum size.Answer:B. It produces the greatest output at a minimum cost