How should the user define the security rule for SSH?
Allow Inbound traffic on port 22 from the user’s network
The user has to create an instance in EC2 Classic with an elastic IP and configure the security group of a private subnet to allow SSH from that elastic IP
The user can connect to a instance in a private subnet using the NAT instance
Allow Inbound traffic on port 80 and 22 to allow the user to connect to a private subnet over the Internet
Explanations:
Allowing inbound traffic on port 22 from the user’s network (IP address or range) is the correct way to allow SSH access to a private instance over a VPN connection, as long as the VPN is connected and the user is in the same network.
EC2 Classic is an old networking model, which is no longer supported. Private instances cannot be accessed using an Elastic IP in EC2 Classic, especially when there is a VPN connection in use.
A NAT instance allows outbound internet traffic from instances in a private subnet but does not facilitate inbound SSH access to those instances. To SSH into an instance in a private subnet, you need a VPN or a Bastion host.
Allowing inbound traffic on ports 80 and 22 does not enable SSH access from the Internet to a private subnet. A private subnet does not have direct Internet access unless it is through a VPN or NAT gateway, which is not the case here.
I estimate that the answer is:
Allow Inbound traffic on port 22 from the user’s network