The right answer is C.
In the ground state, carbon only shows two single electrons. Its tetravalence is explained by the fact that the carbon redistributes two electrons on its L layer, allowing to have four connecting electrons. Raising an electron on a higher electron orbital is called the excitation of an atom.
Methane (CH4) is archetypal of carbon compounds: the tetravalent carbon atom forms four simple covalent bonds with four hydrogen atoms. The carbon atom is called tetravalent because it can establish up to four different single bonds with other atoms or establish, as in the case of alkenes, two single bonds and a double bond, or as in the case of alkynes , a single bond and a triple bond.