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

Question

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

Answer 1

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