In Linux, which of the following is a fast way to copy ssh keys from one server to another?

In Linux, which of the following is a fast way to copy ssh keys from one server to another?

• mv sshkeys.host1 sshkeys.host2
• ssh-keys user@hostname.example.com
• ssh-copy-id user@hostname.example.com
• ssh-copy-keys user@hostname.example.com 

EXPLANATION

The ssh-copy-id command (in the openssh-client package and installed by default) copies ssh key from one server to another.
For example, the following command copies the public key of your default identity to the remote host:
ssh-copy-id user@hostname.example.com