27. Incorrect. The answer is false not true. The efficient price is where price equals marginal cost. Social welfare is the area below demand and above marginal cost

W = ∫₀^QP(Q)dQ - ∫₀^QMC(Q)dQ

When we pick a Q to maximize welfare, we get

∂W/∂Q = ∂ (∫₀^QP(Q)dQ - ∫₀^Q dQ)/∂Q = P(Q) - MC(Q) = 0 => Price equals marginal cost.

Provided second order conditions hold.

∂(P(Q))/∂Q - ∂(MC(Q))/∂Q < 0 or slope of demand is less than slope of marginal cost.