Explanation

(d)
During melting of $SiO_{2},$ the gient network structure held by covalent bonds breaks to give individual molecules of$SiO_{2}.$ In contrast, during boiling of $H_{2}O$ only change of state occurs from liquid to gaseous; during melting of KCN, electrostatic attraction between $K^{+}$ and $CN^{-}$ ions is overcome ; during boiling of $CF_{4},$ van der Waals’ forces of attraction breaks