The wings in a proscenium theatre serve as:

a
The extended portion of the stage that projects towards the audience.
b
The offstage areas on either side of the stage, concealed from the audience's view.
c
The area between the stage and the first row of the audience seating.
d
The seating area for the audience.

b

The offstage areas on either side of the stage, concealed from the audience's view.