The correct option is
D 4
One carbon doesn't form any chain. So there is no chain isomerism in case of alkane having one carbon atom.
CH4 methane has one carbon atom
Two carbons : Two carbon form a chain, but there is only one way to form this chain, so no isomerism is there here as well.
CH3−CH3 ethane has two carbon atoms
Three carbon atoms :
One carbon has to be in the middle and the other two at the ends . So, only one possible chain is there which contains all the three carbon atoms. So, chain isomerism is not there.
Four carbon atoms :
One carbon has to be in the middle, two at the ends, but the fourth carbon could be in the middle (forming 2-methylpropane) or at the end (forming butane).
So , minimum four carbon atoms show chain isomerism.