When the mix of goods and services produced is just what society desires, then exists?

The answer is "allocative efficiency".

Allocative efficiency is a condition of the economy in which good produced speaks to buyer inclinations; specifically, every good or service is delivered up to the point where the last unit gives a negligible advantage to buyers equivalent to the peripheral cost of producing. Allocative efficiency happens where cost is equivalent to marginal cost that is ( P=MC), in light of the fact that cost is society's proportion of relative worth of an item at the margin or its marginal benefit.

answer; i do believe its (b) 12.5 years of age;