Halogenation of alkanes in the presence of sunlight is

The reactions that take place under sunlight occur via radical formation. It is an chain reaction which involves chain initiation, propagation and termination. In the initiation step radicals are formed, in propagation step radicals are propagated to all reacting molecules whereas in termination radicals will react with each other and converted into stable molecules. In case of halogenation of alkanes, halogen radical is generated first which abstract a hydrogen radical from alkanes and generate alkyl radical. The obtained alkyl radical will react with another chlorine radical and produces alkyl halide. Halide radical displaces   hydrogen radical, therefore it is an free radical substitution reaction

Hence the correct option is (D).