[Reference: docs.aws.amazon.com, Delete the NAT Gateway:, Removing the NAT gateway ensures that EC2 instances in the application account cannot access the internet, satisfying the requirement to prevent internet access. This action enhances the security posture by eliminating a potential vector for unauthorized outbound traffic., Create a VPC Peering Connection:, Establishing a VPC peering connection between the application account's VPC and the core account's private VPC enables direct, private communication between the EC2 instances in both accounts. This setup allows the application account's EC2 instances to access the patch source repository hosted in the core account securely., Reference: docs.aws.amazon.com, Update Route Tables in Both Accounts:, After setting up the VPC peering connection, it's crucial to update the route tables in both VPCs to allow traffic to flow between them. This configuration ensures that the EC2 instances in the application account can reach the patch source repository in the core account and vice versa., Why Other Options Are Incorrect:, Option A: Implementing a custom VPN solution introduces unnecessary complexity and operational overhead. Additionally, merely blocking outbound traffic on port 80 does not comprehensively prevent internet access, as other ports (e.g., 443 for HTTPS) remain open., Option B: Creating private virtual interfaces (VIFs) is typically associated with AWS Direct Connect, which is not applicable in this scenario. Moreover, using a transit gateway, while feasible, is more complex and may be unnecessary for this use case., Option D: Blocking inbound traffic on port 80 does not prevent outbound internet access. Furthermore, employing a transit gateway adds complexity and cost, which may not be justified given the requirements., Conclusion:, Option C provides a secure, efficient, and cost-effective solution that meets all the specified requirements:, Prevents EC2 instances from accessing the internet., Enables access to Amazon S3 and Systems Manager services via VPC endpoints., Facilitates secure communication with the patch source repository in the core account through VPC peering., This approach leverages AWS's native networking features to maintain a secure and private environment for patch management operations., ]