1. Bronsted acid-base concept

A Bronsted acid is a proton donor, and a Bronsted base is a proton acceptor.

A species that can act as both acid and base is called amphiprotic.

2. Check each option

Option A: $HSO_4^-$

It can donate a proton: $HSO_4^- \rightarrow SO_4^{2-} + H^+$ So it acts as a Bronsted acid.

It can also accept a proton: $HSO_4^- + H^+ \rightarrow H_2SO_4$ So it acts as a Bronsted base.

Hence, $HSO_4^-$ is both acid and base.

Option B: $Na_2CO_3$

This is a salt. The basic species present is $CO_3^{2-}$.

$CO_3^{2-}$ can accept a proton: $CO_3^{2-} + H^+ \rightarrow HCO_3^-$ So it acts as a base.

But it has no proton to donate, so it does not act as a Bronsted acid.

Hence, $Na_2CO_3$ is not both.

Option C: $NH_3$

It can accept a proton: $NH_3 + H^+ \rightarrow NH_4^+$ So it is a Bronsted base.

Under normal Bronsted consideration, it is not taken as a proton donor here.

Hence, $NH_3$ is not acting as both acid and base in this context.

Option D: $OH^-$

It can accept a proton: $OH^- + H^+ \rightarrow H_2O$ So it acts as a base.

It can also donate a proton: $OH^- \rightarrow O^{2-} + H^+$ But in aqueous acid-base chemistry, $OH^-$ is generally treated as a Bronsted base, and formation of $O^{2-}$ is not feasible in water. Hence it is not usually considered amphiprotic in this level/context.

So this is not the intended correct answer.

3. Conclusion

The species clearly acting as both Bronsted acid and Bronsted base is: $\boxed{HSO_4^-}$

So the correct option is A.