Methane CH4 is molecule with 4 C-H bonds and adopts a tetrahedral geometry.
Now, The valence electron configuration of C = 2s²2p²
H = 1s¹
The electrons are paired in the 2s orbital whereas there exits 2 unpaired electrons in the 2p-orbital.
In order to form four bonds with the 4 H atoms in CH4, we need 4 unpaired electrons. These are created when an electron from the 2s orbital gets promoted to the 2p orbital. This results in the mixing of the 2s and 2p ( three 2p orbitals) forming 4-sp3 hybrid orbitals on the C atom. Each of the four hybrid orbitals then go on to form a sigma bond with each of the four 1s orbital of the H atom.
Ans: (d) The carbon atom promotes one electron from the s orbital into the p orbital, which creates four half-filled orbitals available to accept one electron from four different hydrogen atoms.