Why does the united states use sanctions as a tool of foreign policy?

The United States uses sanctions as a way to punish foreign countries. Sanctions are essentially economic punishments made by one country against another. These punishments can include trade restrictions, tariffs (taxes on imported goods), or trade barriers. By implementing a sanction, one country is hurting another country's economy and financial well being. 

In using these, the US hopes that the country who they punish will change whatever policy the US disagrees with.

The answer should be b.