The goal is to ensure the application operates across multiple Availability Zones. That means removing single points of failure in compute, database, and network egress for private subnets.

For compute, the Auto Scaling group currently has min=1 and max=1, which guarantees only one instance is running at a time. Even if the Auto Scaling group spans multiple subnets, a single-instance configuration cannot provide multi-AZ active capacity because a failure of the Availability Zone hosting the single instance would cause downtime until a new instance is launched in a different zone. To operate across multiple Availability Zones, the solution should run multiple instances across different AZs, which implies increasing the minimum capacity above 1 and allowing the group to launch across multiple subnets/AZs.

For the database, a single-AZ RDS for MySQL instance is a single point of failure. Converting to an RDS Multi-AZ configuration provides synchronous replication to a standby in a different Availability Zone and managed failover to maintain availability when the primary AZ or instance fails.

For NAT, a single NAT gateway is an AZ-scoped managed resource. If private subnets in other AZs route to a NAT gateway in one AZ, an AZ outage can break outbound connectivity for workloads that depend on NAT. The resilient pattern is to deploy a NAT gateway in each Availability Zone and configure each private subnet’s route table to use the NAT gateway in the same AZ.

Option A addresses all three: it deploys additional NAT gateways per AZ and updates route tables accordingly, converts RDS to Multi-AZ, and adjusts Auto Scaling to launch across AZs with min and max set to 3 so there are instances running in multiple AZs simultaneously. This ensures the application remains available across AZ failures, meeting the requirement.

Option B replaces NAT with a virtual private gateway, which is used for VPN/Direct Connect connectivity, not for internet egress from private subnets. It does not satisfy the NAT functionality requirement. Although Aurora can provide high availability, the NAT replacement is not correct for the described architecture.

Option C increases operational overhead by replacing NAT gateway with NAT instances, which are self-managed and less resilient without additional design. It also changes the database engine to PostgreSQL unnecessarily and does not directly address the requirement with minimal change.

Option D improves NAT resiliency but only enables RDS automatic backups, which is a durability feature, not a high availability feature. Backups do not provide automatic failover and do not ensure the application will operate across multiple AZs. Also, keeping Auto Scaling min/max at 1 still leaves the application compute layer single-AZ at any given moment.

Therefore, option A is the correct solution.