You can't call it a social affair exactly, but it's a social event.
A.It is a social event, and it is more important than a social affair.
B.It's not an ordinary social activity for people to meet and communicate with each other,but a very significant social event in which famous people will show up.
C.It's not right for you to call it a social affair because only social event can call it so.
D.You can't talk to him at a social a任air； you can only call him at a social event.
正确答案是B