Concept:The reaction is the chlorination of methane, which is a free-radical substitution reaction initiated by light.Explanation:The reaction between methane and chlorine requires the breaking of the chlorine-chlorine bond to form chlorine free radicals.This bond breaking occurs when chlorine molecules absorb energy from light, especially ultraviolet light.Thus, light is the major factor that provides the activation energy needed to initiate the reaction.Answer:D. Light