The correct choice is Reconfigure the Auto Scaling group to span at least two Availability Zones and register the instances with the existing Application Load Balancer. This option moves instances into separate fault domains and lets the load balancer route traffic to healthy targets if one Availability Zone becomes unavailable.

Reconfigure the Auto Scaling group to span at least two Availability Zones and register the instances with the existing Application Load Balancer improves resilience because Auto Scaling can launch replacements in other zones within the same Region and the Application Load Balancer can distribute traffic across those zones without any application code changes. Using multiple Availability Zones provides high availability at the infrastructure level while keeping the architecture simple.

Create a second Auto Scaling group in a different AWS Region and use Amazon Route 53 failover routing is incorrect because it introduces cross-Region complexity and it requires data replication and failover planning which goes beyond the stated requirement to improve availability without changing application code.

Enable cross-zone load balancing on the existing Application Load Balancer only is incorrect because cross-zone load balancing does not help if all instances are still located in a single Availability Zone. If that AZ fails there will be no healthy targets to receive traffic even with cross-zone balancing enabled.

Add additional subnets from the same Availability Zone to the Auto Scaling group is incorrect because adding more subnets in the same AZ leaves the deployment within a single fault domain and does not protect against an AZ outage.

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