A: Tentacles (tentacular crown is the ring of tentacles )
I'm sure that most of us call the tentacles of a zoanthid as the skirt.
B:Oral Disc
I'm not sure what this area is called but most people call it the center.
C:
This is a there or not type of area. Some zoas have a change in their areas before they reach the mouth, or opening. This area doesn't always exist so it is a zoa to zoa basis.
D: MOUTH
I'm pretty sure that most people call this part the mouth.