Which one of the following is the correct statement ?

Options

A. `HC_(3)^(-)` is the conjugate base of `CO_(3)^(2-)`
B. `NH_(2)^(-)` is the conjugate acid of `NH_(3)`
C. `NH_(3)` is the conjugate base of `NH_(2)^(-)`
D. `H_(2)CO_(3)` is the conjugate base of `HCO_(3)^(-)`

Explanation

To determine which of the given statements is correct regarding conjugate acid-base pairs, we need to analyze each option based on the concept of conjugate acids and bases.

### Step-by-Step Solution:

1. **Understanding Conjugate Acid-Base Pairs**:
- A conjugate acid is formed when a base gains a proton (H⁺).
- A conjugate base is formed when an acid loses a proton (H⁺).
- The difference between a conjugate acid and its base is always one H⁺ ion.

2. **Analyzing Each Option**:
- **Option A**: CH₃⁻ is the conjugate base of CO₃²⁻.
- CH₃⁻ (methyl anion) and CO₃²⁻ (carbonate ion) are different species.
- There is no direct relationship in terms of gaining or losing H⁺.
- **Conclusion**: This statement is incorrect.

- **Option B**: NH₂⁻ is the conjugate acid of NH₃.
- NH₂⁻ (amide ion) is actually a base, and NH₃ (ammonia) is a neutral molecule.
- For NH₂⁻ to become NH₃, it would need to gain an H⁺, making NH₃ the conjugate acid of NH₂⁻, not the other way around.
- **Conclusion**: This statement is incorrect.

- **Option C**: NH₃ is the conjugate base of NH₂⁻.
- NH₃ (ammonia) cannot be a conjugate base of NH₂⁻.
- NH₂⁻ is a base, and when it gains an H⁺, it becomes NH₃, which means NH₃ is the conjugate acid of NH₂⁻, not the base.
- **Conclusion**: This statement is incorrect.

- **Option D**: H₂CO₃ is the conjugate base of HCO₃⁻.
- H₂CO₃ (carbonic acid) can lose a proton to form HCO₃⁻ (bicarbonate ion).
- This means HCO₃⁻ is indeed the conjugate base of H₂CO₃.
- **Conclusion**: This statement is correct.

3. **Final Answer**:
- The correct statement is **Option D**: H₂CO₃ is the conjugate base of HCO₃⁻.