Electronic configuration of four elements A, B, C and D are given below :

(a) $1s^{2}2s^{2}2p^6$  (b) $1s^{2}2s^{2}2p^4$

(c) $1s^{2}2s^{2}2p^{6}3s^1$  (d) $1s^{2}2s^{2}2p^5$

Which of the following is the correct order of increasing tendency to gain electron?

(A) $1s^{2}2s^{2}2p^6$:

Total electrons: 2 + 2 + 6 = 10 It is an electrical configuration made up entirely of group 18 element Neon. The electron gain enthalpy of this element is positive.

(B)$1s^{2}2s^{2}2p^4$: 

The electrical arrangement is for the element oxygen, which is in group 16. High negative electron gain enthalpy is present in this element.

(C) $1s^{2}2s^{2}2p^{6}3s^1$:

The group 1 element, sodium, has the following electrical configuration. Low negative electron gain enthalpy characterises this element.

(D)$1s^{2}2s^{2}2p^5$:

The group 17 element fluorine has the following electrical arrangement. Among the given configurations, this element has the highest negative electron gain enthalpy.
As a result, for the above configurations, the increasing order of electron gain enthalpies is:

A < C < B < D.