The suggested answer is D.
In an active/passive high availability (HA) configuration for Palo Alto Networks Next-Generation Firewalls (NGFWs), the active firewall is given a lower numerical priority value to ensure it takes precedence. Given that the active firewall has a priority value of 100, the passive firewall must be assigned a higher numerical priority value to ensure it does not take over unless necessary. Therefore, the correct priority for the passive firewall should be a value greater than 100, making 255 the appropriate choice.