One of the following is a Bronsted acid but not a Bronsted base:

Options

A. `H_(2)S`
B. `H_(2)S`
C. `HCO_(3)^(Theta)`
D. `NH_(3)`

Explanation

To solve the question "One of the following is a Bronsted acid but not a Bronsted base," we need to analyze the given compounds and determine their behavior as Bronsted acids and bases.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- A **Bronsted acid** is a substance that donates protons (H⁺ ions).
- A **Bronsted base** is a substance that accepts protons (H⁺ ions).

2. **Analyze Each Compound**:
- **Option A: H₂S (Hydrogen Sulfide)**:
- H₂S can dissociate to release H⁺ ions:
\[ \text{H}_2\text{S} \rightarrow \text{HS}^- + \text{H}^+ \]
- It can donate protons, thus acting as a Bronsted acid.
- H₂S does not readily accept protons to form a stable compound, so it does not act as a Bronsted base.

- **Option B: H₂S (again)**:
- This is the same as Option A, so the analysis is identical. H₂S is a Bronsted acid but not a Bronsted base.

- **Option C: HCO₃⁻ (Bicarbonate Ion)**:
- HCO₃⁻ can dissociate to release H⁺ ions:
\[ \text{HCO}_3^- \rightarrow \text{CO}_3^{2-} + \text{H}^+ \]
- It can also accept a proton to form carbonic acid (H₂CO₃):
\[ \text{HCO}_3^- + \text{H}^+ \rightarrow \text{H}_2\text{CO}_3 \]
- Therefore, HCO₃⁻ acts as both a Bronsted acid and a Bronsted base.

- **Option D: NH₃ (Ammonia)**:
- NH₃ can accept a proton to form NH₄⁺:
\[ \text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+ \]
- It can also dissociate to release H⁺ ions:
\[ \text{NH}_3 \rightarrow \text{NH}_2^- + \text{H}^+ \]
- Thus, NH₃ acts as both a Bronsted acid and a Bronsted base.

3. **Conclusion**:
- From the analysis, the only compound that acts as a Bronsted acid but not a Bronsted base is **H₂S**.

### Final Answer:
**H₂S (Hydrogen Sulfide)** is a Bronsted acid but not a Bronsted base.