Which of the following commands preloads and manages keys that are used for automatic authentication while logging in to other machines using SSH?

`sshd` is the OpenSSH server daemon, responsible for listening for and handling incoming SSH connections.

`ssh-agent` is a program that runs in the background and stores private keys in memory, acting as an authentication agent. It makes these keys available to SSH clients for automatic authentication to remote servers without requiring the passphrase for each connection.

`ssh-keygen` is used to generate, manage, and convert authentication keys for SSH.

`ssh-add` is used to add identity (private key) files to the authentication agent (`ssh-agent`); it works in conjunction with `ssh-agent` but `ssh-agent` is the managing process.