What does the command mount -a do?

A.
It mounts the floppy disk for all users.

B.
It shows all mounted file systems.

C.
It opens /etc/fstab to edit.

D.
It mounts all file systems listed in /etc/fstab.

Explanation/Reference:
From the man pages:
-a, –all Mount all filesystems (of the given types) mentioned in fstab.