Answer:
An acid because they donate H^+ ions
Step-by-step explanation:
Let us look at the equation again;
H2SO3 + H2O « H3O+ + HSO3-
An acid donates hydrogen ion while a base accepts hydrogen ion. Hence, we have to look at which species that acted as hydrogen ion donors or acceptors.
We can clearly see that H2SO3 acted as a hydrogen ion donor while H2O acted as a hydrogen ion acceptor.
Thus, H2SO3 acted as an acid by donating hydrogen ions.