You are authoring a CloudFormation template to deploy a web service on EC2 instances in a private subnet behind an Application Load Balancer (ALB). The service must accept requests from IPv6 clients. What changes should you make in the template so IPv6 clients can reach the service?

To allow IPv6 clients to reach a service behind an ALB, the ALB itself must be configured for IPv6. This involves adding an IPv6 CIDR block to the VPC and the subnets where the ALB is deployed. Crucially, the ALB's IP address type must be set to dualstack to enable both IPv4 and IPv6 communication. The ALB then forwards requests to the EC2 instances, which can still operate with IPv4 internally. The first incorrect option is wrong because while adding IPv6 to the VPC and subnets for EC2 instances is good practice, it doesn't directly enable the ALB to accept IPv6 client connections. The second option is incorrect because Elastic IPs are primarily for public IPv4 addresses, and assigning them to EC2 instances directly bypasses the ALB's role in handling IPv6 client traffic. The third option is incorrect because replacing the ALB with an NLB is unnecessary; ALBs support IPv6. Also, an NLB would require additional configuration for HTTP/HTTPS traffic that an ALB handles natively.