Final answer:
To reach the social optimum, the government can use subsidies to match the per-unit spillover benefits or by regulating prices and quantities, ensuring that the marginal social benefit equals the marginal social cost.
Step-by-step explanation:
The social optimum, which refers to the ideal level of output where the marginal social benefit (MSB) equals the marginal social cost (MSC), can be achieved through various government interventions. When there are positive externalities, such as with vaccinations, a subsidy equivalent to the per-unit spillover benefits can lead to an increase in the market equilibrium quantity to the socially optimal quantity (Qsocial) and the socially optimal price (Psocial). In the context of fuel-efficient cars, a subsidy that lowers the production cost can increase the equilibrium quantity to Qsocial. Regulators can also set prices and quantities, such as requiring a firm to produce at a quantity where the marginal cost intersects with the demand curve. These measures are intended to correct market failures and ensure that the value consumers get from the last unit consumed is equal to the cost of producing it, thus maximizing social welfare.