Which MPLS control protocol should be protected with MD5 authentication to prevent rogue LSRs from being introduced into the cloud?

LDP (Label Distribution Protocol) is the correct answer because it is responsible for distributing labels between LSRs (Label Switch Routers) in an MPLS network. Protecting LDP with MD5 authentication prevents unauthorized LSRs from participating in label distribution, which could lead to traffic misdirection or denial-of-service attacks. RSVP (Resource Reservation Protocol) is used for establishing explicit LSPs and can also use authentication, but LDP is the primary protocol for label distribution in most MPLS deployments. MP-BGP (Multiprotocol BGP) is used for exchanging VPN routes and other advanced MPLS services, not for basic label distribution. "ALSO" is not a recognized MPLS control protocol.