The correct answer is B. hydrogen abstraction.
In free radical halogenation of hydrocarbons, such as methane, the reaction proceeds through three steps: initiation, propagation, and termination. In the propagation step, a hydrogen atom is abstracted from the hydrocarbon molecule by the halogen radical, forming a new carbon-centered radical and a hydrogen halide molecule.
The bond dissociation energy (BDE) of the C-H bond in methane is 105 kcal/mol, while the BDE of the C-Cl bond in chloromethane is 83 kcal/mol and the BDE of the C-Br bond in bromomethane is 68 kcal/mol.
Because the C-H bond in methane is stronger than the C-Cl and C-Br bonds, it requires more energy to break, making hydrogen abstraction in bromination slower than in chlorination. This is because the C-Br bond is weaker than the C-Cl bond, so the energy released from the reaction of Br with H is lower than that released from the reaction of Cl with H.