If the administrator was able to log in to the remote server, it implies that port 22 is open and SSH is functioning correctly. Therefore, the issue is likely not with the firewall settings.
Option D, “Ensuring the private key was properly imported”, could indeed be the next step. If the administrator is trying to establish a connection using key-based authentication, any issues with the private key (such as it not being properly imported or not being in the correct location) could prevent the connection.