The correct option is
A Bond Energy ∝1Stability of free radicals
Stability of free radicals∝+I +R−I −R
In compound (a), the group
CN shows -R effect which will withdraw the electron density toward itself and destabilise the free radical formed.
In compound (b) and (c), we have
Cl group. Halide groups have more negative inductive effect compare to the resonance effect. Hence, it will withdraw the electron density toward itself and destabilise the free radical formed.
In compound (c),
OCH3 will show +R effect and increase the electron density on carbon and stabilise the free radical formed.
Comparing (a), (b) and (c), compound (a) form least stable free radical due to the resonance effect of
CN
Since, least stable free radical will have highest bond energy, compound (a) has the highest bond energy for
C−Ha bond.