Saturate hydrocarbon like alkane will undergo substitution reaction in presence of sunlight.
CH4+ Cl2 ……………> CH3Cl + HCl.
so option b is correct. in presence of sunlight

Chlorine Reacts with Saturated Hydrocarbons in the Presence of Sunlight

Introduction
Chlorine is a highly reactive element that readily undergoes chemical reactions with various substances. One such reaction is with saturated hydrocarbons, which are organic compounds consisting of only carbon and hydrogen atoms bonded together by single covalent bonds. The reaction between chlorine and saturated hydrocarbons is known as chlorination.

The Role of Sunlight
Chlorine reacts with saturated hydrocarbons at room temperature in the presence of sunlight. Sunlight provides the necessary energy to initiate the reaction between chlorine and hydrocarbons. Without sunlight, the reaction proceeds at a much slower rate or may not occur at all. This is because the energy provided by sunlight breaks the relatively strong carbon-hydrogen (C-H) bonds in the hydrocarbon and allows the reaction to proceed.

The Mechanism of the Reaction
The reaction between chlorine and saturated hydrocarbons involves the substitution of hydrogen atoms in the hydrocarbon molecule with chlorine atoms. This process is known as substitution reaction or halogenation. The reaction proceeds through a free radical mechanism, which involves the following steps:

1. Initiation: In the presence of sunlight, chlorine molecules (Cl2) are dissociated into highly reactive chlorine radicals (Cl·) through homolytic cleavage of the Cl-Cl bond. This homolytic cleavage results in the formation of two chlorine free radicals, each having an unpaired electron.

2. Propagation: The chlorine free radicals react with the saturated hydrocarbon, abstracting a hydrogen atom from the hydrocarbon molecule. This leads to the formation of a hydrogen chloride molecule (HCl) and a hydrocarbon radical. The hydrocarbon radical can then react with another chlorine molecule, leading to the formation of a new chlorine radical and another hydrogen chloride molecule. This process continues until all the hydrogen atoms in the hydrocarbon are replaced by chlorine atoms.

3. Termination: The reaction terminates when two radicals combine to form a stable molecule or when all the hydrogen atoms in the hydrocarbon have been replaced by chlorine atoms.

Conclusion
In conclusion, chlorine reacts with saturated hydrocarbons at room temperature in the presence of sunlight. The energy provided by sunlight initiates the reaction by breaking the carbon-hydrogen bonds in the hydrocarbon. This reaction proceeds through a free radical mechanism, involving the substitution of hydrogen atoms in the hydrocarbon with chlorine atoms. The reaction is important in various industrial processes, such as the production of chlorinated solvents and the chlorination of drinking water to disinfect it.