(i) Electronic distribution of element A and D.
K L M
A = 2,8
D = 2,8,3
(ii) The number of electrons in the outermost shell of element A is 8. The outermost shell of this element is complete and the element does not gain or lose electrons to complete its outermost shell. Hence, A is an inert gas.
(iii) Valency of element C (2, 8, 2) is 2.