What should the company do to achieve this in the shortest amount of time?
Use a blue-green deployment with a complete application-level failover test
Use the RDS console to reboot the DB instance by choosing the option to reboot with failover
Use RDS fault injection queries to simulate the primary node failure
Add a rule to the NACL to deny all traffic on the subnets associated with a single Availability Zone
Explanations:
A blue-green deployment involves significant infrastructure changes and is not a quick solution for failover testing.
Rebooting the DB instance with the failover option initiates a failover to the standby instance, which simulates an Availability Zone failure without requiring code changes.
RDS fault injection queries simulate database issues but are not designed to simulate an Availability Zone failure or automatic failover.
Modifying NACLs to deny traffic would block communication to the DB instance, but it would not simulate an Availability Zone failure or initiate a failover.
As I understand it, the answer is:
Use the RDS console to reboot the DB instance by choosing the option to reboot with failover