Which of the following steps could resolve the issue?

Explanations:

Disabling the Source/Destination Check on the NAT instance allows it to route traffic from the private subnet to the internet and back. This is necessary for NAT instances as they need to handle traffic that is not originating from themselves.

Attaching an Elastic IP address to the instance in the private subnet is not possible since Elastic IPs can only be associated with instances in a public subnet. This option would not resolve the connectivity issue.

Attaching a second Elastic Network Interface (ENI) to the NAT instance and placing it in the private subnet is not appropriate, as the NAT instance needs to have its public interface to route traffic to the internet. The NAT instance should have one interface in the public subnet.

Attaching a second ENI to the instance in the private subnet and placing it in the public subnet does not apply correctly since this option suggests modifying the private instance rather than the NAT instance. The private subnet instance should use the NAT instance for outbound traffic, not have a public ENI itself.