Chlorination of methane proceeds by a free radical substitution mechanism that has three distinct phases: initiation, propagation, and termination. In initiation, $C{l}_2$ undergoes homolytic fission under UV light to generate two chlorine radicals, $C{l}_2\to 2C{l}^{\bullet }$. The propagation steps are the heart of the chain: a chlorine radical abstracts a hydrogen atom from methane to give $C{H}_3^{\bullet }$ and $HCl$, and the methyl radical then reacts with another $C{l}_2$ molecule to give $C{H}_{3}Cl$ and regenerate $C{l}^{\bullet }$. Because $C{l}^{\bullet }$ is consumed and regenerated, it is the species that carries the chain forward. The chloride ion $C{l}^{-}$ would be involved in ionic, not radical, chemistry, so it is wrong. The chloronium ion $C{l}^{+}$ appears in electrophilic addition to alkenes, not in alkane substitution, so it is incorrect. Molecular $C{l}_2$ is the reagent but does not itself propagate the chain; it only supplies radicals after homolysis. This mirrors the NCERT treatment of substitution reactions of alkanes. A consistency check: only a radical mechanism explains why traces of light or peroxide dramatically accelerate the reaction.