Which agency is primarily responsible for preventing terrorists and weapons of mass destruction from entering the country?

The correct answer is that the Border Patrol is primarily responsible for preventing terrorists and weapons of mass destruction from entering the country. The Border Patrol, which is part of the Customs and Border Protection agency, focuses specifically on monitoring and securing the U.S. borders. Their mission includes detecting and preventing unlawful entries, which encompasses efforts to thwart the potential infiltration of terrorists and dangerous materials into the United States.

While other agencies listed play significant roles in overall national security and law enforcement, their functions are more specialized or do not focus solely on border security. The Immigration and Customs Enforcement agency primarily deals with immigration enforcement and customs violations within the country, rather than on the border itself. The Federal Bureau of Investigation focuses on broader criminal investigations, including terrorism, but operates within the country's interior after threats have penetrated the borders. The Transportation Security Administration is tasked with securing transportation systems, particularly air travel, which is critical but operates after potential threats may have already crossed the border.

Thus, the Border Patrol’s specific mandate to control and secure the U.S. borders makes it the central agency in the context of preventing terrorists and weapons of mass destruction from entering the country.