Socially optimal consumer surplus is represented by

• A. The triangle above MC=0
• B. The rectangle under price
• C. The area below demand and above price
• D. The triangle above the point where MC=0

Answer: (D)

Socially optimal consumer surplus comes from the extra value consumers gain when production occurs at the efficient quantity, where price equals marginal cost. On a typical diagram with demand and a rising marginal cost, this welfare is seen as a triangular region formed between the demand curve and the baseline where marginal cost is zero, up to the socially optimal quantity. In other words, it’s the triangle lying above the MC = 0 reference and below the demand curve up to the point where P = MC. This captures the total willingness to pay beyond zero production cost for exactly the units produced at the socially efficient level.