Answer:
a. impose a tax on rain barrels that is equal to the per-unit externality. Explanation:
The rain barrel is necessity and required to meet the scarcity of water during dry areas. This clearly is a major task to regulate the use of rain barrels, as wastage of water of rain barrels will create a problem. For this the government can take various steps to regulate the use of rain barrels.
For this if government puts taxes on excess use of rain barrels then it would impact people's thinking as there will be a cost associated with it.
Thus, correct option is:
Statement a.