(A) Helium is lighter than air, so used in airship balloons. (B) Argon is completely inert to other elements, so used as inert atmosphere. (C) Due to larger size of Xe, the outer electron and nucleus attraction is weak. So, it reacts with highly electronegative element, i.e. fluorine and oxygen. (D) Radon is formed from the natural decay of uranium and radium, so, it is radioactive. The correct match is A-IV, B-I, C-III and D-II.