What should a solutions architect do to resolve this issue?

Explanations:

Disabling source/destination checks on the EC2 instances running the proxy software allows these instances to forward traffic that is not directly addressed to them. By default, EC2 instances perform source/destination checks, which prevent them from routing traffic that is not intended for their own IP addresses. Disabling this feature enables the proxy to properly forward traffic to and from the internet.

While adding a rule to allow traffic between instances with the same security group may improve internal communication, it does not resolve the issue of forwarding traffic to the internet. The problem lies in the routing and source/destination checks, not internal security group settings.

Changing the VPC’s DHCP options set to point the DNS server options to the addresses of the proxy EC2 instances does not directly resolve the issue of traffic routing. The proxy needs to handle the forwarding of traffic rather than merely serving as a DNS resolver. This option does not address the core requirement for traffic forwarding.

Assigning an additional elastic network interface (ENI) to each proxy EC2 instance may allow for multiple network connections, but it complicates the setup without necessarily resolving the routing issue. Proper traffic routing and handling depend on configuring the instances correctly and disabling source/destination checks rather than simply adding more interfaces.