Aviation and Airport Security Practice Test.

The USCIS (United States Citizenship and Immigration Services) is the primary enforcement agency for protecting the borders. This is because their main responsibility is to administer and enforce immigration laws and policies, including inspecting individuals who are seeking to enter the United States.

While the other options may play a role in protecting the borders, they do not have the primary responsibility for immigration and border control. The FBI focuses on criminal investigations and counterterrorism, the CIA focuses on intelligence gathering and international affairs, and the DEA focuses on drug enforcement. Therefore, the USCIS is the most relevant and appropriate response for this question.