Final answer:
The TSA is responsible for protecting U.S. ports of entry, as part of their mission to secure national transportation systems.
Step-by-step explanation:
The U.S. federal agency responsible for protecting the U.S. ports of entry is the Transportation Security Administration (TSA). As part of the Department of Homeland Security, the TSA has duties that include safeguarding transportation systems to ensure freedom of movement for people and commerce.
While agencies like the FBI and CIA have important roles in law enforcement and international intelligence, respectively, the security of ports of entry falls under the TSA as they oversee the security of the nation's transportation systems. The DEA focuses on enforcing the controlled substances laws and regulations of the United States and would not typically be involved in port security unless it pertains to drug trafficking operations.