Functional group present in sulphonic acids is :

In organic chemistry, a sulphonic acid is defined as a compound having the general formula $$R{-}SO_3H$$, where $$R$$ is an alkyl or aryl group.

The functional group responsible for the characteristic properties of these acids is therefore the $$-SO_3H$$ group (one sulphur atom doubly bonded to two oxygen atoms, singly bonded to a hydroxyl group $$(-OH)$$, and singly bonded to the carbon skeleton).

Among the given options, only Option B represents this group.

Hence, the functional group present in sulphonic acids is:

Option B which is: $$-SO_3H$$