The correct option is D (34)n
Let X=x1,x2,....xn
For each xi∈X(1≤i≤n), we have the following four choices
(1)xi∈A and xi∈B(2)xi∈A and xi/∈B(3)xi/∈A and xi/∈B(4)xi/∈A and xi/∈B
Thus the number of ways of choosing A and B is 4n. Out of the occurrence of A∪B=X. Therefore, the number of favourable ways is 3n.
Hence, the probability of the required event is (34)n