Explanation
A and D are the best practice deployment modes for the firewall if the company wants to add threat prevention to the network without redesigning the network routing. This is because these modes allow the firewall to act as a transparent device that does not affect the existing network topology or routing1.
A: VirtualWire mode allows the firewall to be inserted into any existing network segment without changing the IP addressing or routing of that segment2. The firewall inspects traffic between two interfaces that are configured as a pair, called a virtual wire. The firewall applies security policies to the traffic and forwards it to the same interface from which it was received2.
D: Layer 2 mode allows the firewall to act as a switch that forwards traffic based on MAC addresses3.
The firewall inspects traffic between interfaces that are configured as Layer 2 interfaces and belong to the same VLAN. The firewall applies security policies to the traffic and forwards it to the appropriate interface based on the MAC address table3.
Verified References:
1: https://www.garlandtechnology.com/blog/whats-your-palo-alto-ngfw-deployment-plan
2:
https://docs.paloaltonetworks.com/pan-os/10-1/pan-os-admin/networking/configure-interfaces/virtual-wire
3:
https://docs.paloaltonetworks.com/pan-os/10-1/pan-os-admin/networking/configure-interfaces/layer-2.htm