The correct answer is: Decommission the existing NAT instances and deploy two NAT gateways in separate Availability Zones.

Explanation:
Amazon NAT Gateways are managed AWS services designed to provide scalable, highly available internet access for resources in private subnets. By placing NAT gateways in multiple Availability Zones, you achieve fault tolerance and high availability. AWS automatically handles scaling and redundancy, eliminating the need for manual instance management or load balancing.

Why the other options are incorrect:

Deploying two NAT gateways in the same Availability Zone does not provide fault tolerance. If that AZ becomes unavailable, internet connectivity for all dependent resources is lost.

Using Auto Scaling groups with Network Load Balancers to manage EC2 NAT instances increases complexity and requires manual configuration and management. While possible, it lacks the simplicity and automation of NAT gateways and still carries single points of failure without careful design.

Replacing NAT instances with EC2 Spot Instances introduces risk, as Spot Instances can be terminated with little notice. This is inappropriate for critical infrastructure services like NAT, which must be stable and reliable.

Why this question is important: It evaluates your knowledge of networking, availability, and managed service options in AWS. NAT Gateway is a common topic on the AWS exams and demonstrates how to improve reliability while reducing operational overhead.

Tip: For highly available, scalable internet access from private subnets, always favor NAT Gateways in multiple AZs over EC2 NAT instancesespecially as traffic grows.

Full AWS Practitioner Certification Question