Question

name the opening through which water leaves the body cavity of sponge

Answer 1

Water leave the body of sponge through a large pore known as "Osculum"

Answer 2

The answer of your question is "Osculum" 
It's the structure in the sponges through which water leaves the body cavity of sponge. Actually it's a quite large opening , the water current after passing through a structure "spongocoel" leaves the body of sponge by the osculum. The  average velocity by which water diffuse as well as exit through osculum is 8.4 cm per second. This water also carries away all the wastes of sponge during exit.