What does the term "proscenium" refer to in theatre design?

Question

a
The arch or rectangular opening between the stage and the audience seating area in a proscenium theatre.
b
The extended portion of the stage that projects towards the audience.
c
The raised stage and semicircular seating area in ancient Greek theatre.
d
The enclosed space that separates the performers and the audience.

Answer 1

a
The arch or rectangular opening between the stage and the audience seating area in a proscenium theatre.