Network Load Balancer is the correct choice because it operates at Layer 4 and it preserves the client source IP address and source port while scaling to tens of millions of concurrent TCP connections.

Network Load Balancer forwards raw TCP connections so backend Nginx instances observe the original client IP and port without relying on X-Forwarded-For. It is designed for ultra low latency and extreme connection scale which fits a mobile gaming API that must handle tens of millions of simultaneous TCP connections.

Application Load Balancer is incorrect because it is a Layer 7 load balancer and it conveys client details via headers such as X-Forwarded-For rather than preserving the source port, and it is optimized for HTTP and HTTPS routing rather than raw TCP at extreme concurrency.

Classic Load Balancer is incorrect because it is a legacy option that is not intended for tens of millions of concurrent connections and it does not natively preserve both client IP and port without additional workarounds like proxy protocol.

Elastic Load Balancer is incorrect because that phrase refers to the overall ELB service family rather than a specific load balancer implementation, so it is ambiguous and does not directly satisfy the requirement.

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