Final answer:
To disable password authentication for SSH, you must edit the 'PasswordAuthentication' setting in the 'sshd_config' file to 'no' and restart the SSH service.
Step-by-step explanation:
To disable password authentication so that an SSH key is required, you need to edit the SSH daemon configuration file on the server. This file is typically called sshd_config and is located in the /etc/ssh directory. You should look for the line that specifies PasswordAuthentication and change its value to 'no'. After making this change, you will need to restart the SSH service for the changes to take effect. Doing this will ensure that password authentication is disabled and only key-based authentication is allowed, enhancing the security of your SSH server.
The correct way to disable password authentication is option c: Edit the sshd_config file and set PasswordAuthentication to 'no'. After saving the changes, remember to restart your SSH server with a command like sudo systemctl restart ssh on most Linux distributions.