For a government subsidy on a good with an external benefit
For a government subsidy on a good with an external benefit to result in the efficient amount of output being produced, what must be done?
|   The size of marginal external benefit must be accurately  determined. | ||
|   The government must produce the product. | ||
|   Private production and private consumption must both be directly  subsidized. | ||
|   The quantity demanded must be decreased to the efficient  amount. | ||
|   Private production without the subsidy must be prohibited. | 
Solution
Ans A is right answer. When size of marginal external benefit is accurately determined we can equate it with marginal cost So as to determine optimum quantity. All other options are wrong. 100% sure about answer. Please please like answer

