How does the ionization energy of elements in Group $1$ change as we move down the group?

$KO_2$ (Potassium superoxide) is used in confined spaces and submarines because it

Comment on each of the following observations:
$(a)$ The mobilities of the alkali metal ions in aqueous solution are $Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$
$(b)$ Lithium is the only alkali metal to form a nitride directly.
$(c)$ The standard electrode potential $E^{\circ}$ for the reaction $M^{2+}_{(aq)} + 2e^{-} \to M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

Given below are two statements:
Statement $I$: Lithium and Magnesium do not form superoxide.
Statement $II$: The ionic radius of $Li^{+}$ is larger than the ionic radius of $Mg^{2+}$.
In the light of the above statements,choose the most appropriate answer from the options given below:

Select the correct order :-

Which of the following does not liberate a brown gas?