Answer: a. less than the socially optimal price, greater than the socially optimal quantity
Explanation:
The steel mill is producing steel and selling at a rate that does not account for the pollution that it is causing. Because of this, it is selling at a lower equilibrium price than what it would had the Pollution been accounted for.
The Steel Mill is also selling quantity that is greater than what would be considered socially optimal because the socially optimal level would account for the pollution and adjust in such a way that the Pollution is minimized