At the socially optimum quantity of production price equals...?

Question

At the socially optimum quantity of production price equals...

a.marginal cost

b.average total cost

c.average variable cost

d.marginal revenue

AND WHY?

I believe it's marginal cost.

Answer 1

I think "B" should be right.

Because it allows companies to stay in business in long-run and at the same time maximizes their output thus maximizing consumer surplus.