For the reaction  2H2+O2⟶2H2O,ΔH=−571 Bond energy H-H = 435, 0 = 0 = 498, then calculate the average bond energy of O - H bond using the above data.

Reaction  2H2+O2⟶2H2O;ΔH=−571 We know that  ∆H(reaction) = Bond energy of reactantsbond energy of reactants- ∴ −571=2× B.E H2+ B.E. O2−(2× B.E. of H2O         −571=2×435+498−2× B.E. of H2O;−571=870+498−2B⋅E⋅H2O2× B.E. of H2O=870+498+571=1939∴ B.E. of H2O or H−O−H=19392=969.5∴ B.E. of OH bond =969.52=484.75∝484So, the correct answer is (a)