Which reactions involves a change in the electron `-` pair geometry `( i.e.,` hybridisation ) for the under lined geometry ?

Options

A. `underset(bar)(B)F_(3)+F^(-)rarr underset(bar)(B)F_(4)^(-)`
B. `underset(bar)(N)H_(3)+H^(+)rarr underset(bar)(N)H_(4)^(+)`
C. `2underset(bar)(S)O_(2)+O_(2)rarr2underset(bar)(S)O_(3)`
D. `H_(2)underset(bar)(O)+H^(+)rarrH_(3)underset(bar)(O)^(+)`

Explanation

To determine which reaction involves a change in electron pair geometry (or hybridization), we will analyze the hybridization states of the reactants and products in each given reaction.

### Step-by-Step Solution:

1. **Identify the Reactions**: The question provides several reactions. We need to analyze each one to see if there is a change in hybridization.

2. **Analyze the First Reaction**:
- Reaction: H₂O + H → H₃O⁺
- Hybridization of H₂O: The oxygen in water (H₂O) is sp³ hybridized because it has four electron pairs (two bonding pairs and two lone pairs).
- Hybridization of H₃O⁺: In hydronium ion (H₃O⁺), the oxygen is also sp³ hybridized (three bonding pairs and one lone pair).
- **Conclusion**: No change in hybridization.

3. **Analyze the Second Reaction**:
- Reaction: 2 S₂O + O₂ → 2 SO₃
- Hybridization of S₂O: The sulfur in S₂O is sp² hybridized.
- Hybridization of SO₃: In sulfur trioxide (SO₃), sulfur is also sp² hybridized.
- **Conclusion**: No change in hybridization.

4. **Analyze the Third Reaction**:
- Reaction: NH₃ + H⁻ → NH₄⁺
- Hybridization of NH₃: Ammonia (NH₃) has nitrogen that is sp³ hybridized.
- Hybridization of NH₄⁺: The ammonium ion (NH₄⁺) also has nitrogen that is sp³ hybridized.
- **Conclusion**: No change in hybridization.

5. **Analyze the Fourth Reaction**:
- Reaction: BF₃ + F⁻ → BF₄⁻
- Hybridization of BF₃: Boron trifluoride (BF₃) has boron that is sp² hybridized.
- Hybridization of BF₄⁻: In the tetrafluoroborate ion (BF₄⁻), boron is sp³ hybridized.
- **Conclusion**: There is a change in hybridization from sp² to sp³.

6. **Final Conclusion**: The only reaction that involves a change in hybridization is the fourth reaction (BF₃ + F⁻ → BF₄⁻). Therefore, the correct answer is option A.

### Summary:
- The reaction that involves a change in electron pair geometry (hybridization) is **BF₃ + F⁻ → BF₄⁻**.