What should the solutions architect recommend?
Remove the two NAT instances and replace them with two NAT gateways in the same Availability Zone.
Use Auto Scaling groups with Network Load Balancers for the NAT instances in different Availability Zones.
Remove the two NAT instances and replace them with two NAT gateways in different Availability Zones.
Replace the two NAT instances with Spot Instances in different Availability Zones and deploy a Network Load Balancer.
Explanations:
Using two NAT gateways in the same Availability Zone does not provide high availability or fault tolerance. If the Availability Zone fails, traffic will be disrupted.
While Auto Scaling groups with Network Load Balancers can provide scalability, NAT instances are not designed to work with Auto Scaling groups effectively due to stateful nature. This option would not meet the requirements.
Replacing the NAT instances with NAT gateways in different Availability Zones offers high availability, fault tolerance, and automatic scaling. NAT gateways are managed services that automatically scale with traffic and are resilient to Availability Zone failures.
Spot Instances are not suitable for NAT functions due to their potential for termination and lack of guaranteed availability. This would introduce risk rather than providing a highly available and fault-tolerant solution.
I deduce that the answer is:
Remove the two NAT instances and replace them with two NAT gateways in different Availability Zones.