The correct solution is Modify the security group for the EC2 instances to allow traffic exclusively from the security group associated with the ALB. In AWS, security groups can reference other security groups as sources. By allowing traffic only from the ALB's security group, the EC2 instances are effectively protected from all other trafficwhether from within the VPC or from the internet. This setup restricts access to the EC2 instances and ensures only traffic passing through the ALB can reach them.

The option Add a new route to the private subnet's route table that sends external traffic directly to the EC2 instances' private IP addresses is incorrect. Route tables direct traffic at the subnet level but do not control access. Additionally, exposing private IPs to external sources via routing violates the principle of keeping these instances private and secure.

The choice Move the EC2 instances to the public subnet and assign them Elastic IP addresses to restrict traffic undermines the entire purpose of having a private subnet. Making the instances publicly accessible increases their exposure to potential threats, and it doesnt restrict access to only the ALB.

The option Update the ALB's security group to allow unrestricted TCP traffic from all sources and ports weakens the security posture rather than strengthens it. It opens up the ALB to broad access, but does nothing to protect the EC2 instances from other internal traffic sources.

This question highlights an important concept in AWS security: using security group references to tightly control traffic flow within a VPC. When faced with a scenario involving controlled access from a specific AWS resource (such as an ALB), referencing the source's security group in the target's security group is a best practice. This provides a dynamic, tightly-scoped security model thats easy to manage.

The subtle challenge in this question lies in recognizing that subnets and route tables dont offer the granularity needed for source-based restrictionssecurity groups are the right tool for the job.

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