Root bridge election in Spanning Tree Protocol is governed entirely by the Bridge Identifier, an 8-byte value formed by concatenating the configured bridge priority (the higher-order, more significant portion of the comparison) with the switch's MAC address (used strictly as a tiebreaker when priorities are identical). Every participating switch compares Bridge IDs, and the switch advertising the numerically lowest Bridge ID is elected root. In this exhibit, the three switches have distinctly different priority values — EX-1 at the Junos/IEEE default of 32768, EX-2 at 8192, and EX-3 at 16384 — meaning the comparison is decided entirely at the priority stage and never needs to proceed to a MAC address tiebreak at all. Since 8192 is numerically the lowest of the three priority values, EX-2 is elected root bridge purely on the strength of its lower priority, regardless of what its MAC address happens to be. This is precisely why the correct answer cites priority rather than MAC address as the deciding factor: MAC address only becomes relevant as a tiebreaker in the specific case where two or more switches share an identical priority value, which is not the situation presented here. Understanding that priority is evaluated first and is independently sufficient to decide the outcome whenever it differs is a foundational and heavily tested Spanning Tree concept on the JNCIS-ENT exam. Reference topics: Junos Enterprise Switching – Spanning Tree Protocols, Bridge ID and Root Bridge Election.