Lets take example let A={1,2}
Q can be 0 element set, then no of 0 element sets =1 .. Then its subsets possible ie P
=2^0 =1
Q can be 1 element set, no of such sets s =2 .. no of subsets in each = 2^1
Q can be 2 element sets =1..
no of subsets possible =2^2
Total combinations=1+4+4=9
Since correct option must be true for all cases including 2 element set , correct ans is 3 ^n