Which of the following exists as a zwitter ion ?

Sulfanilic acid (p-aminobenzenesulfonic acid) contains both an acidic group ($-SO_3H$) and a basic group ($-NH_2$).

The strongly acidic sulfonic acid group donates a proton to the basic amino group, resulting in the formation of an internal salt or a dipolar ion known as a zwitterion.

The structure of the zwitterion is $p\text{-}^+H_3N-C_6H_4-SO_3^-$.

The other given compounds do not possess a combination of a sufficiently strong acidic group and a basic group to form a stable zwitterion.

Answer: Sulfanilic acid