In Bluetooth security, which security mode enforces the link-level security?

• A. Security mode 1 (non-secure)
• B. Security mode 2 (service level enforced security)
• C. Security mode 3 (link level enforced security)
• D. Security mode 4 (firmware-level security)

Answer: (C)

Link-layer protection means the connection is authenticated and encrypted at the Bluetooth link level before any higher-layer data is exchanged. The mode that enforces this behavior requires the pairing process to occur and the link to be encrypted before any L2CAP channels or services can communicate. This ensures confidentiality and integrity from the moment a connection is established, preventing eavesdropping or tampering on the link itself. The other modes do not enforce this level of protection: one mode applies security only when accessing a service after the link is up, so the link itself could be unprotected; another mode offers no security at all; and a fourth option describes firmware-level security rather than a standard link-layer enforcement mechanism.