What does the command mount -a do?

What does the command mount -a do?

A.
It ensures that all file systems listed with the option noauto in /etc/fstab are mounted.

B.
It shows all mounted file systems that have been automatically mounted.

C.
It opens an editor with root privileges and loads /etc/fstab for editing.

D.
It ensures that all file systems listed with the option auto in /etc/fstab are mounted.

E.
It ensures that all file systems listed in /etc/fstab are mounted regardless of their options.