c
\((c)\) In the unhybridized state of carbon, \(2 p\) orbitals are \(90^o\) to one  another and each one will overlap with \(1s\) orbital of three hydrogen atoms, thus  three \(C-H\) bonds are formed which are \(90^o\) to one another. For the fourth  hydrogen atom, its \(1s\) orbital may overlap with \(non-\) directional \(2s\)  orbital  of the carbon and this \(\sigma\) -bond will be stronger than \(\sigma C-H\) bonds  formed by \(2p-1s\) overlap. In such situation \(CH_4\) molecule can never has  tetrahedral geometry.