The correct solution is: Deploy the RDS database as a Multi-AZ instance and create a separate subnet in each Availability Zone. Set the Auto Scaling group to launch instances across both subnets.

This design ensures high availability by distributing EC2 compute resources across multiple Availability Zones and enabling Multi-AZ deployment for the RDS database. In an Auto Scaling group, specifying subnets from different Availability Zones allows Amazon EC2 to launch instances in a zone-independent manner. RDS Multi-AZ, meanwhile, maintains a standby replica in another zone and automatically fails over in case of a zone outageproviding fault tolerance at the data layer.

The option Create a single subnet that spans multiple Availability Zones. Distribute EC2 instances through the Auto Scaling group across this subnet and configure the RDS instance with Multi-AZ settings is invalid because subnets are region-specific and can only reside in one Availability Zone. There is no such thing as a subnet that spans AZs in AWS. This is a common misconception and a trick frequently seen in exam questions.

The option Add two subnets that extend across both Availability Zones. Update the Auto Scaling group to spread instances across the zones, and connect the RDS instance to each subnet is flawed. First, subnets cannot extend across Availability Zones. Second, RDS does not use subnet connections for failoverMulti-AZ is a feature that must be enabled explicitly to create a standby in a second zone.

The option Set up one subnet per Availability Zone. Update the Auto Scaling group to use both subnets. Modify the RDS instance to connect to each of the subnets, but keep it in a single Availability Zone improves EC2 availability but keeps the database in a single point of failure. This still violates the goal of full high availability.

This question is significant because it tests your understanding of AWS Availability Zone isolation and how to architect resilient applications using services like Auto Scaling and RDS. A key trick in questions like this is to watch for wording that violates AWS design principles, like subnets "spanning" AZs or RDS connecting to "multiple networks." These are subtle but critical flaws in the incorrect choices.

This question is challenging because it combines multiple layers of high availabilitynetwork, compute, and databaseand requires you to understand how each AWS component contributes to fault tolerance. Look for solutions that span zones and use native AWS features like Multi-AZ to ensure redundancy.

Full AWS Practitioner Certification Question