Explanation

(a)
(i) As we go down in group, in group. I, ionisation potential decreases and dissociation $(M - OH)$ bond becomes easier.
(ii) The hydroxide which can give $OH^{-}$ ion most easily will have highest basicity.
$\therefore$ Ionisation energy of Cs is least among Li, Na, K, Cs
$\therefore$ CsOH furnishes $OH^{-}$ most easily.
$\therefore$ CsOH is most basic.