The correct answer is C and D.
C: Fix the file permissions with read-only access to the owner of the file.
D: Open port 22 access for the computer's public IP address.
The authorized_keys file on the server should have read-only access for the owner of the file, and no access for anyone else. This ensures that only the owner can read the public keys that are authorized to log in, and no one can modify or delete them. The file permissions can be fixed with the command chmod 400
~/.ssh/authorized_keys on the server. This is a recommended security practice for SSH key-based authentication123.
The computer that wants to log in to the server using SSH key-based authentication needs to have access to port 22 on the server, which is the default port for SSH service. This can be done by opening port 22 access for the computer's public IP address on the server's firewall or security group settings. This allows the computer to initiate an SSH connection to the server and authenticate with its private key. Opening port 21, which is used for FTP service, is not relevant or secure for SSH key-based authentication1.