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 that listens for incoming SSH connections, not a client-side tool for managing authentication keys.

The `ssh-agent` is a background program that stores private keys in memory, allowing them to be used for SSH authentication multiple times during a session without repeatedly prompting for their passphrases.

`ssh-keygen` is used to generate, manage, and convert authentication keys for SSH, but it does not preload or manage them for a running session.

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