Concept:The reaction between hydrogen and chlorine is a photochemical chain reaction that becomes explosive in the presence of intense light.Explanation:In sunlight, chlorine molecules absorb energy and break into free radicals.These chlorine radicals attack hydrogen molecules, producing hydrogen chloride and more radicals.The chain reaction proceeds very rapidly in direct sunlight, releasing a large amount of energy and causing an explosion.In diffused or weak light, the reaction is slow and controlled, not explosive.Answer:The correct option is sunlight.