The C-H bond distance is longest in: i) C2H2 ii) C2H2Br2 iii) C2H4 iv) C2H6

Bond length decreases with increase in bond multiplicity. In C₂H₄ and C₆H₆ there exists double bond. In C₂H₂ triple bond exists. In C₂H₄Br₂ single bond exists.