Answer: Create a network load balancer that used backend services containing one instance group with two instances.

**A: Create a network load balancer that uses backend services containing one instance group with two instances.** **Correct.** This refers to an **external passthrough Network Load Balancer** (formerly called TCP/UDP Network Load Balancer), which supports external IPv4 and IPv6 via separate forwarding rules. Use a backend service with a **regional managed instance group** (spanning multiple zones) containing at least two instances for true high availability (automatic distribution and failover across zones). This is the modern, recommended approach for simple TCP passthrough with HA, preserving client source IP, and minimal latency. It's ideal for non-HTTP/TCP workloads needing direct external exposure. **B: Create a network load balancer that uses a target pool backend with two instances.** **Incorrect (legacy).** Target pools are the older backend type for classic Network Load Balancers. While functional, they are less flexible (no support for autoscaling MIGs, limited health checking options, zonal only by default). Google recommends migrating to backend services with instance groups for new designs. **C: Create a TCP proxy that uses a zonal network endpoint group containing one instance.** **Incorrect.** TCP Proxy (external proxy Network Load Balancer) supports zonal NEGs (GCE_VM_IP_PORT type), but a single-instance zonal NEG provides no redundancy or high availability. Even if scaled to multiple endpoints, TCP Proxy terminates connections (adds proxy hop/latency) and is better for global anycast or cross-region scenarios — not the primary choice here when passthrough NLB meets the requirements. **D: Create a TCP proxy that uses backend services containing an instance group with two instances.** **Incorrect (suboptimal for this scenario).** TCP Proxy does support backend services with instance groups (managed or unmanaged) as valid backends — the original explanation was wrong to claim it requires NEGs exclusively. However, TCP Proxy is still not the best fit: it proxies/terminates TCP (unlike passthrough NLB), may introduce unnecessary latency, and is typically chosen for global load balancing or when features like SSL offload are needed. For regional HA with simple TCP external access on a custom port + IPv4/IPv6, the passthrough Network Load Balancer (Option A) is preferred. **Reference Answer: A** The recommended approach is **Option A** — an external passthrough Network Load Balancer using a backend service with a regional instance group (ideally managed, with instances across zones) for high availability, direct TCP passthrough, and native IPv4/IPv6 support via dual forwarding rules. This aligns with modern GCP best practices for the described requirements.