Suppose Nauru is a small country that can import "rice" at the world price. There is a positive production externality from domestic rice production that producers cannot appropriate. Specifically,this means thateach unit of rice produced yields a marginal external social benefit. Which of the following policies should be implemented by Nauru to address this market failure if the goal is to maximize social welfare?:

a. A tariff on rice imports

b. An export subsidy for rice exports

c. A quota on rice imports

d. A production subsidy for domestic rice production

Which option is correct?