The energy involved in the conversion of 21C2(g) to Cl−1(aq) is given by ΔH=21ΔdissHCl2(−)+ΔegHCl(−)+ΔhyIHCl(−) Substituting various values from given data, we get ΔH=(21×240)+(−349)+(−381)kJmol−1=(120−349−381)kJmol−1=−610kJmol−1i.e., the correct answer is (b)