In Palo Alto Networks firewalls, when multiple static routes exist for the same destination, the firewall uses the administrative distance (AD) to determine route priority. The AD is a metric that indicates the trustworthiness of a route, with lower values indicating higher priority. For static routes, the default AD is 10, but this can be manually adjusted. The route with the lowest AD is preferred and added to the routing table. If AD values are equal, the firewall then considers the metric (default 10), but AD is the primary differentiator.
Option A (first route installed) is incorrect, as route installation order does not determine priority. Option C (Bidirectional Forwarding Detection) is a protocol for detecting link failures, not route priority. Option D (highest AD) is the opposite of the correct behavior. This aligns with standard routing principles and Palo Alto's implementation.
Reference: Palo Alto Networks PAN-OS 11.2 Administrator's Guide, "Networking" section - Virtual Routers and Routing Protocols.