What must be done on a host to allow a user to log in to that host using an SSH key?

What must be done on a host to allow a user to log in to that host using an SSH key?

A.
Add their private key to ~/. ssh/authorized_keys

B.
Reference their public key in ~/. ssh/config

C.
Run ssh-agent on that host

D.
Add their public key to ~/. ssh/authorized_keys

E.
Reference their private key in ~/. ssh/config