Correct Answer - C
In ethyne, central C-atom is sp hybridised and triple bond is present between two carbon atoms. Hence the bond length is small.
C-H bond in ethyne is sp-s `sigma`-bond, in ethane it is `sp^2-s-sigma` bond and in ethene it is `sp^3-s-sigma` and sp-hybrid orbital being smallest in size the sp-s-`sigma` bond will be shortest
