c
\((c)\) Only in \(BF_3\, \,p \pi - p \pi\) back bonding is possible due to which \(B-F\)  bond length is shortest. In other compounds, \(p-\) orbital at boron is not vacant hence, \(p \pi - p \pi\) back bonding is not possible. Water molecules are  hexagonally closed pocked in ice with the help of \(H-\) bonds, hence due to existance of voids/interstitial spaces it has cage like structure with less density  than water.