What have been secured to some extent by limiting access to underground areas?

Transportation terminals—such as airport cargo areas, seaports, and train yards—have implemented access controls that limit entry to underground or restricted zones. These security measures help prevent unauthorized access and protect critical infrastructure and high-value cargo.

Maritime operations (A) refer to port activities but not specifically to underground security.

Security measures (B) is too general.

Resealing (D) is not relevant to underground access control.