Which of the following commands is used to hold keys during a login session to be used for automatic authentication while logging in to other machines using ssh?

`sshd` is the OpenSSH daemon, which listens for incoming SSH connections, not manages keys for outgoing connections.

`ssh-agent` is a program that manages private keys and makes them available to SSH clients, allowing users to unlock their private keys once per session and then use them for multiple SSH connections without re-entering the passphrase. It acts as an intermediary, providing authentication details securely.

`ssh-keygen` is used to generate, manage, and convert authentication keys, not to hold them during a session.

`ssh-add` is used to add private key identities to the authentication agent (`ssh-agent`), but `ssh-agent` itself is the program that holds the keys.