Let S be the universal set and (n)X = n. The probability of selecting two subsets A and B of the X such that $B =\bar{A}$ is:

$\frac{1}{2}$ $\frac{1}{2^n-1}$ $\frac{1}{2^n}$ $\frac{1}{3^n}$

$\frac{1}{2^n}$

X is the universal set. Total number of ways of selecting 2 sets A and B. So two choices for each of elements ⇒ Total number of favorable cases = 2n Probability = $\frac{2^n}{2^n×2^n}=\frac{1}{2^n}$