Remember that a Brønsted-Lowry acid is a molecule or ion that donates a hydrogen ion in a reaction and a Brønsted-Lowry base is a molecule or ion that accepts a hydrogen ion in a reaction.
In this case, you can see that ammonium ion is donating hydrogen to the compound HSO3 (-).
NH4 (+) acts as Bronsted-Lowry acid because it releases a proton (H) to become NH3. The answer is 2.