What happens on sulphonation of C_6H_5Cl ?

The correct answer is option 4. o- and p-chlorobenzene sulphonic acid is formed.

Chlorine is an ortho/para director, so the sulfonation will occur at the ortho and para positions of the benzene ring. This will produce a mixture of o-chlorobenzenesulphonic acid and p-chlorobenzenesulphonic acid.

The other options are incorrect. Option (1), m-chlorobenzenesulphonic acid, is not formed because chlorine is not a meta director. Option (2), benzene sulphonic acid, is not formed because the sulfonation reaction is selective and will only occur at the ortho and para positions of the benzene ring. Option (3), o-chlorobenzene sulphonic acid, is not the only product formed.