on TCP port 8080. Which feature must be enabled on the virtual server for this configuration to work?

HTTP persistence tracks client sessions for load balancing continuity but does not alter the destination port of forwarded connections.

Source NAT (SNAT) modifies the source IP address of outgoing connections so backend servers can route replies back through the BIG-IP, and does not affect destination port mapping.

Destination address affinity is a persistence method that pins a client to the same pool member based on destination IP, and has no role in port number translation.

Port translation is the feature that allows a BIG-IP virtual server to receive client traffic on a specific port (such as 80) and then re-write the destination port to a different value (such as 8080) before forwarding the connection to the pool member. Without port translation enabled, the virtual server would forward traffic to the same port it received it on, making it impossible to redirect to port 8080 on the backend.