Answer:
a. a subsidy so that the firm can operate where marginal social benefit equals marginal social cost.
Explanation:
The private company is producing when the marginal revenue matches the marginal cost. The governemtn will want to decrease the cost (that's by subsidize the activity) to match the marginal revenue considering the positive externalities.
The government will do a pigouvian subsidy.
The government reasons to go for this is that the good or services provide positive externalities Which are enjoy by people who doens't purchase the good. Thus, this subsidy will increase the amount of ooutput thus, generating a better social benefit.