Question

Why is the reaction between methane and chlorine in the presence of sunlight considered as a substitution reaction?

Answer 1

Methane Has 4atoms of Hydrogen.
Chlorine one-by-one replaces hydrogen and substitutes it (comes in its place).
Therefore it is Substitution Reaction

Answer 2

When sunlight is provided as a catalyst to the halogens, the halogens affine to have a homolytic dislocation and form free radicals

Now, these halogens free radicals tend to interact with the stable alkanes and form their radicals too
So, this forms a loop of reactions in which free radicals go on forming and this is also known as propogation step.
Ultimately, this loop reaction stops and the reaction terminates due to one of the limited reagent.

Here, substitution of the halogens occurs in alkanes to form Alkyl halides and thus these reactions are known as to be substitution reactions