A company needs to configure a NAT instance for its internal AWS applications to be able to download patches and package software. Currently, they are running a NAT instance that is using the floating IP scripting configuration to create fault tolerance for the NAT. What is the best way to configure the NAT instance with fault tolerance?

Answer options:

A.Create one NAT instance in a public subnet and create a route from the private subnet to the NAT instance.
B.Create two NAT instances in a public subnet and create a route from the private subnet to each NAT instance for fault tolerance.
C.Create a NAT instance in a public subnet with application running in private subnet in an AZ. Create a similar architecture in another AZ and create a route from the private subnet to each NAT instance residing in these AZ`s for fault tolerance.
D.Create two NAT instances in two separate private subnets.

Answer – C
Option A is incorrect because you would need at least two NAT instances for fault tolerance.
Option B is incorrect because if you put both NAT instances in a single public subnet and that subnet becomes unavailable or unreachable to the other instances, the architecture would not be fault tolerant.
Option C is CORRECT because you should place two NAT instances in two separate public subnets, and create routes from instances via each NAT instance for achieving fault tolerance.
Option D is incorrect because you should not be putting the NAT instances in the private subnet as they need to communicate with the internet. They should be in the public subnet.
More information on NAT instances:
One approach to this situation is to leverage multiple NAT instances that can take over for each other if the other NAT instance should fail. This walkthrough and associated monitoring script (nat_monitor.sh) provide instructions for building an HA scenario where two NAT instances in separate Availability Zones (AZ) continuously monitor each other. If one NAT instance fails, this script enables the working NAT instance to take over outbound traffic and attempts to fix the failed instance by stopping and restarting it.
Below is a diagram for fault tolerant NAT instances.
For more information on fault-tolerant NAT gateways, please see the below link-
https://aws.amazon.com/articles/2781451301784570