The species which can act both as Bronsted acid and base is

Options

A. `CO_3^2-`
B. `NO_3^-`
C. `HSO_4^-`
D. `SO_4^2-`

Answer

To determine which species can act both as a Brønsted acid and a Brønsted base, we need to understand the definitions of these terms:

1. **Brønsted Acid**: A substance that donates a proton (H⁺).
2. **Brønsted Base**: A substance that accepts a proton (H⁺).

Now, let's analyze the given species:

1. **CO₃²⁻ (Carbonate ion)**:
- Can accept a proton to form HCO₃⁻ (bicarbonate), so it can act as a Brønsted base.
- However, it does not have any protons to donate, so it cannot act as a Brønsted acid.

2. **NO₃⁻ (Nitrate ion)**:
- Similar to carbonate, it can accept a proton to form HNO₃ (nitric acid), thus acting as a Brønsted base.
- It does not have protons to donate, so it cannot act as a Brønsted acid.

3. **SO₄²⁻ (Sulfate ion)**:
- It can accept a proton to form HSO₄⁻ (hydrogen sulfate), so it can act as a Brønsted base.
- It does not have protons to donate, so it cannot act as a Brønsted acid.

4. **HSO₄⁻ (Hydrogen sulfate ion)**:
- It can donate a proton to form SO₄²⁻ (sulfate), thus acting as a Brønsted acid.
- It can also accept a proton to form H₂SO₄ (sulfuric acid), thus acting as a Brønsted base.

From the analysis, we can see that only **HSO₄⁻** can act both as a Brønsted acid and a Brønsted base.

### Final Answer:
The species which can act both as a Brønsted acid and base is **HSO₄⁻ (Hydrogen sulfate ion)**.