A critical application is deployed in two AWS Regions and uses an Application Load Balancer in each Region. Route 53 alias records point to both ALBs, and Route 53 Application Recovery Controller (ARC) provides routing controls for failover testing. During a quarterly DR test both routing controls were accidentally turned off. The company wants to guarantee that at least one routing control is always ON. Which solution enforces that constraint?

The correct answer is to create an assertion safety rule with type ATLEAST and a threshold of 1. An assertion safety rule is designed to enforce a specific state across a set of routing controls. By setting the type to ATLEAST and the threshold to 1, the rule ensures that at least one of the specified routing controls must always be in the ON state, preventing both from being accidentally turned off simultaneously. Gating safety rules are used to block changes based on the state of other routing controls, not to enforce a minimum number of active controls. Resource sets and readiness checks are used to verify the readiness of resources for failover, not to enforce operational constraints on routing control states.