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 SSH daemon, which runs on the server to listen for and accept incoming SSH connections, not to hold client authentication keys.

`ssh-agent` runs in the background and stores decrypted private keys in memory, making them available for SSH clients without requiring the user to type the passphrase each time. This provides convenience and enhanced security by preventing keys from being left unprotected on disk for frequent use.

`ssh-keygen` is used to generate, manage, and convert authentication keys for SSH, not to hold them in an agent 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 and manages these keys.