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Paramjeet verma
Option (A) more the electronegative group attached to the C-C , more the tendency to pull towards itself the electrons forming the C-C bond. CN group pull towards itslef the electrons in C-C bond and reduces its bond length. Option (B) and option (C) also have CN group but aslo on the other C atom is sp and sp² hybridised (more s-character) and thus electronegative. So the electrons are pulled towards both sides and thus the overall effect gets cancelled.