

We have *: p(X) * p(X) -> p(X)
Given, A * B = A ∩ B
So, X * A = X ∩ A = A for all A
Hence, X is an identity element.
Again, let I is another identity.
So, I ∩ A = A for all A
and x ∈ X, I ∩ {x} = {x}
=> x ∈ I
=> X ⊂ I and I ⊂ X
=> I = X
So, X is the identity element.
