A subnet must reside within a single Availability Zone. https://aws.amazon.com/vpc/faqs/#:~:text=Can%20a%20subnet%20span%20Availability,within%20a%20single%20Availability%20Zone.

a subnet only resides on a one AZ, it does not span to another AZ.

A subnet can't "Extend" across multiple AZs: B,D out HA = RDS Multi-AZ: A out C

An Auto Scaling group can span across two Availability Zones, where one subnet is created in each AZ. When creating an Auto Scaling group, you need to specify at least one subnet. You can add additional subnets later on, including subnets across multiple AZs. Auto Scaling will distribute instances evenly across the specified subnets to maintain availability and optimize performance. If one AZ becomes unavailable, instances can be launched in the other AZ. The associated load balancer should also span the same subnets/AZs as the Auto Scaling group. This allows traffic to be routed to instances in different subnets and AZs, increasing fault tolerance of the application.

Provision a subnet in each Availability Zone. Configure the Auto Scaling group to distribute the EC2 instances across both Availability Zones. Configure the DB instance for Multi-AZ deployment

This solution will ensure that the EC2 instances and the DB instance are not located in the same Availability Zone, which will improve the availability of the application.

D is completely wrong, because each subnet must reside entirely within one Availability Zone and cannot span zones. By launching AWS resources in separate Availability Zones, you can protect your applications from the failure of a single Availability Zone.

The key word here was extend.

This discards B and D: Subnet basics. Each subnet must reside entirely within one Availability Zone and cannot span zones. By launching AWS resources in separate Availability Zones, you can protect your applications from the failure of a single Availability Zone

a subnet is per AZ. a scaling group can span multiple AZs. https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-add-availability-zone.html

I think D. Span the single subnet in both Availability Zones can access the DB instances in either zone without going over the public internet.

it's C