The correct option is C Tertiary > Secondary > Primary
For photochemical halogenation, the rate of the reaction is directly proportional to the stability of the intermediate formed.
The intermediates formed in photochemical halogenation is free radical.
Free radicals are stabilised by hyperconjugation effect.
More the number of α−Hydrogen more will be the stability of radicals. Thus tertiary radicals have more α−Hydrogen than secondary radical and primary have least α−Hydrogen. Thus, stability of free radicals follows the order,
Tertiary radical > Secondary radical > Primary radical
Therefore, the reactivity of hydrogen atoms attached to carbon atom in the halogenation of an alkane has the order,
Tertiary > Secondary > Primary
Thus, option (c) is correct.