Bases are the substance which give $-OH$ ions (hydroxyl ions) when dissolved in water. If one molecule of a base gives two hydroxyl ions, it is called diacidic base.
(i) $CH _{2}( OH )_{2}$ is a covalent compound and $C - O$ bond is more stronger than $O - H$ bond. Hence, it dissociates to give only a less amount of $H ^{+}$ ions instead of $OH ^{-}$ ion and acts as acid.
(ii) $Ca ( OH )_{2}$, being an ionic compound readily gives two hydroxyl ions when dissolved in water, hence it is a diacidic base.
$\underset{\text{diacidic base}}{Ca ( OH )_{2}} \longrightarrow Ca ^{2+}+2 OH ^{-}$
(iii) $CH _{3} CH ( OH )_{2}$ is also a covalent compound and gives a small amount of $H ^{+}$ ions, hence it is an acid.
Actually when two $- OH$ groups are attached with same carbon atoms, the compound is highly unstable and loses a water molecule, eg,
