Given an environment where incoming traffic for VMSS1 must be distributed across two network virtual appliances (NVA1 and NVA2) with minimal administrative overhead, which load-balancing solution should you recommend?

Gateway Load Balancer is the correct choice because it's specifically designed for transparently inserting network virtual appliances (NVAs) like firewalls or intrusion detection systems into the network path. It operates at Layer 3/4 and uses a "bump-in-the-wire" model, ensuring all traffic flows symmetrically through the NVAs without requiring any changes to the application or source/destination IPs. This minimizes administrative overhead. Azure Front Door is a global, Layer 7 load balancer for web applications, not suitable for transparently inserting NVAs at lower layers. Azure Application Gateway is also a Layer 7 load balancer, primarily for HTTP/S traffic, and doesn't provide the transparent NVA insertion capability. Azure Traffic Manager is a DNS-based traffic management service, directing traffic to different endpoints but not acting as an inline load balancer for NVAs.