If A∩B=B, then
A⊆B
B⊆A
A=B
None of these
Let x∈A⇒x∈A∪B, [∵A⊆A∪B]
⇒x∈A∩B, [∵A∪B=A∩B]
⇒x∈A and x∈B⇒x∈B, ∴A⊆B
Similarly, x∈B⇒x∈A, ∴B⊆A
Now A⊆B, B⊆A⇒A=B.