Which agency is primarily responsible for customs enforcement in the United States?

The agency that is primarily responsible for customs enforcement in the United States is Customs and Border Protection (CBP). This federal agency is part of the Department of Homeland Security and plays a crucial role in regulating and facilitating international trade, collecting import duties, and enforcing U.S. trade laws. CBP officers are tasked with preventing illegal goods from entering the country, protecting the borders, and ensuring compliance with customs regulations.

In this context, other agencies do not have a primary focus on customs enforcement. The Department of Commerce primarily deals with regulations and policies that affect economic growth and trade, while the Department of Justice is concerned with enforcing the law and administering justice in the United States. The Environmental Protection Agency focuses on environmental protection and regulation, which, while important, is not directly related to customs enforcement. Thus, Customs and Border Protection's dedicated mission makes it the correct answer.