Which of the following indicates the path over which an LSP's tail end router will return an RSVP RESV message?

The RSVP PATH message is sent from the ingress (head-end) to the egress (tail-end) router to establish the path for an LSP. As the PATH message traverses each router, it creates a Path State Block (PSB). The PSB stores crucial information, including the address of the previous hop from which the PATH message was received. When the egress router generates an RSVP RESV (Reservation) message, it sends it upstream. Each router uses the previous-hop information stored in its PSB to forward the RESV message back along the exact reverse path toward the ingress router.

A. The IGP FIB: The IGP Forwarding Information Base is for standard IP routing and is not used to forward RESV messages, which must follow the specific reverse LSP path.
B. The Reservation State Block (RSB): The RSB is created by the RESV message itself as it travels upstream; it cannot be used to guide the message that creates it.
D. The opaque database: This database stores TE link attributes for CSPF path calculation by the IGP; it is not involved in forwarding RSVP messages.

Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide, Release 21.10.R1, Part: RSVP, Section: RSVP-TE Operation. This section describes how PATH messages create path state (PSB) and RESV messages follow this state in reverse.

RFC 2205, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", Section 3.1.2, "Path State". This RFC specifies that "Each node along the path stores the identity of the previous hop... This information is then used to route Resv messages hop-by-hop in the reverse direction."