The Protocol Independent Multicast (PIM) Bootstrap Router (BSR) election is determined by the BSR priority value. The candidate router with the numerically highest priority is elected as the BSR for the domain. In this scenario, Router B is configured with a priority of 100, which is the highest value among all candidates (Router A: 10, Routers C & D: default 0). Therefore, Router B is elected as the BSR. The IP address is only used as a tie-breaker if two or more routers share the same highest priority value, which is not the case here.

A. Router A is not elected because its BSR priority of 10 is lower than Router B's priority of 100.

C. Router C is not elected because its default priority of 0 is lower than the priorities of Router A and Router B.

D. Router D is not elected because the highest IP address is a secondary tie-breaker, and its lower priority (0) disqualifies it from the primary election criteria.

1. IETF RFC 5059, "Bootstrap Router (BSR) Mechanism for PIM": Section 3.1, "BSR Election", states: "The BSR election is based on the BSR priority field that is included in the Bootstrap message. A C-BSR with a higher BSR priority is preferred over a C-BSR with a lower BSR priority. If the BSR priorities are equal, then the C-BSR with the higher BSR IP address is preferred."

2. Nokia 7750 SR OS R21.10.R1 Services Guide: In the "PIM" chapter, section "Bootstrap Router", it is specified that "The BSR is elected from the candidate BSRs. The election is based on the BSR priority. If the priorities are equal, the BSR with the highest IP address is elected." The guide also confirms the default priority is 0.