What must also be enabled when using source NAT if the address pool is in...

When source NAT uses a pool of addresses from the same subnet as the egress interface , the firewall must respond to ARP requests for those NAT pool IPs. Without this, upstream devices would not know how to forward traffic destined for those IPs.

Proxy ARP is required (Option D). It enables the SRX to answer ARP requests on behalf of the NAT pool addresses.

Static NAT (Option A) is unrelated and maps one-to-one, not required here.

Dynamic DNS (Option B) has no relation to NAT pools.

Destination NAT (Option C) applies to inbound translations, not outbound source NAT pools.

Correct Feature: Proxy ARP