Identify the best suitable reagent for the following reaction.

Halogen atoms are ortho- para directing but deactivating groups(due to $-\mathrm{I} \mathrm{effect}$).

The ortho product is the minor product and the para product is the major product. In presence of Concentrated sulphuric acid chlorobenzene undergoes aromatic electrophilic substitution reaction to form 2-Chlorobenzenesulphonic acid and 4-chlorobenzenesulphonic acid(major product), where the electrophile is sulphurtrioxide

Reaction:-

Hence, concentrated Sulphuric acid is the best suitable reagent for the above reaction.