The order of decreasing C-C bond lengths in the following molecules 

(i) C₂H₄       (ii) C₂H₂        (iii) C₆H₆      (iv) C₂H₆

The correct order of decreasing C−C bond length is IV > III > I > II.

IV: Ethane - has all single bonds making the longest bond lengths.
III: Benzene - has resonance meaning that the C-C bond has equal single bond and douible bond character.
II: Ethyne - has a triple bond
I: Ethene: has a double bond
More the number of bonds, shorter the bond length.$C-C\to 1.54\stackrel{0}{A}$

$C=C\to 1.36\stackrel{0}{A}$

$C\equiv C\to 1.20\stackrel{0}{A}butin$

$C_6{H}_6,$ $C=1.39\stackrel{0}{A}$