(a) Among the given elements, D(19) has one electron in the outermost shell. Its electronic configuration is 2, 8, 8, 1.
(b) Among the given elements, A(4) and E(20) have two electrons in their respective outermost shells. Electronic configuration of A: 2, 2 Electronic configuration of E: 2, 8, 8, 2 They both have a valency of two. Thus, they belong to group 2 of the periodic table.
(c) Among the given elements, A(4) and B(9) belong to period 2 and elements D(19) and E(20) belong to period 4 of the periodic table. Also, as the atomic radii of elements decrease on going from left to right in a period because of increase in the effective nuclear charge that pulls the outermost electron closer to the nucleus, A(4) has a bigger atomic radius than B(9) and D(19) has a bigger atomic radius than E(20).