EC2 instances in private subnets cannot send traffic directly to the internet gateway because private subnets do not have a direct 0.0.0.0/0 route to the internet gateway. Instead, outbound IPv4 internet access from private subnets is typically provided by a NAT gateway that is deployed in a public subnet. Private subnet route tables then send default internet-bound traffic (0.0.0.0/0) to the NAT gateway as the target. The NAT gateway uses its Elastic IP address and the public subnet’s route to the internet gateway to enable outbound connectivity while still preventing inbound internet-initiated connections to the private instances.

When the company reduced multiple NAT gateways to a single NAT gateway, the routing design likely became inconsistent across Availability Zones. In multi-AZ architectures, it is common to have one NAT gateway per AZ and have each private subnet route to the NAT gateway in the same AZ to maintain zonal resiliency and avoid cross-AZ dependencies. If NAT gateways were removed, some private subnets may still have route table entries that point to NAT gateways that no longer exist. Those subnets would immediately lose outbound internet connectivity. The fix is to update the route tables associated with the affected private subnets so that their 0.0.0.0/0 route targets the remaining NAT gateway.

Option C would violate the requirement that instances must be in private subnets; routing private subnet traffic directly to an internet gateway effectively makes the subnet public. Option A is unnecessary and adds operational burden; NAT instances require patching, scaling, and availability management. Option D does not address the routing target problem and is not how NAT gateway egress is restored for multiple subnets.

Therefore, correcting the private subnet route tables to send internet-bound traffic to the existing NAT gateway is the appropriate solution.

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