Q.Which of the following can act as a Lowry–Bronsted acid as well as a base?

Step 1. HCl has no proton left to accept once it loses its one $H^+$ (forming the unstable $H_2Cl^+$ is not the intended answer here) and readily behaves only as an acid -- not amphiprotic in the ordinary sense tested here.

Step 2. $SO_4^{2-}$ has no further proton to donate (it is already fully deprotonated); it can only accept a proton (to form $HSO_4^-$) -- base only.

Step 3. $Br^-$ is the conjugate base of the strong acid HBr and has essentially no basic character and no proton to donate -- neither a useful acid nor base here. …

Check whether each species has BOTH a proton to donate and a lone pair/negative charge able to accept one -- only a species in the mid …

• Picking SO4^2- because it is a common Bronsted base, overlooking that it has no proton left to …