Socially optimal consumer surplus on a monopoly graph is the triangle above

• A. The point where MC = 0
• B. The point where MR = 0
• C. The triangle above the point where MC = 0
• D. The triangle below the price line

Answer: (C)

Socially efficient output happens where price equals marginal cost, and consumer surplus at that point is the area under the demand curve and above that price, up to the efficient quantity. On the monopoly diagram, that region forms the triangle bounded by the demand curve, the MC-based price line at the efficient quantity, and the vertical axis up to that quantity. Since the MC level that supports the efficient quantity defines the baseline where MC crosses zero, the relevant consumer surplus is the triangle above that MC = 0 point. The other descriptions don’t match the welfare-maximizing region: using MC = 0 alone isn’t the welfare-maximizing price, MR = 0 relates to profit terms rather than welfare, and the triangle below the price line would not capture the full consumer surplus at the efficient quantity.