Answer:
c. The equilibrium quantity is less than the socially optimal quantity.
Step-by-step explanation:
Externalities are positive / negative side effects to other parties, which are not monetarily valued & compensated.
Positive Externalities cause extra positive side effect, have extra social benefit apart from private benefit. Their free market unregulated equilibrium under estimates their Total Benefit (considering only private benefit , ignoring social benefit). So the equilibrium quantity is also under estimated. Hence, Equilibrium quantity is less than socially optimal quantity.