(b)
When two$- OH$ groups are attached with same carbon atoms, the compound is highly unstable and loses a water molecule,
e.g.,
$$
$Ca(OH)_{2}$, being an ionic compound readily gives two hydroxyl ions when dissolved in water, hence it is a diacidic base.
$Ca(OH)_{2}\ \rightarrow Ca^{2 +} + 2OH^{-}$
Diacidic base