Assertion : Bond dissociation energy of `B-F` bond in `BF_(3)` molecule is lower than `C-F` bond in `CF_(4)` molecule.
Reason : Atomic size of B-atom is larger than that of C-atom.

Options

A. If assertion is true but the reason is false
B. If assertion is false but reason is true
C. If both assertion and reason are true and the reason is the correct explanation of assertion
D. If both assertion and reason are true and the reason is not the correct explanation of assertion

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

**Assertion**: The bond dissociation energy of the B-F bond in BF₃ is lower than that of the C-F bond in CF₄.

**Reason**: The atomic size of the B-atom is larger than that of the C-atom.

### Step-by-Step Solution:

1. **Understanding Bond Dissociation Energy**:
- Bond dissociation energy is the energy required to break a bond in a molecule. A higher bond dissociation energy indicates a stronger bond.

2. **Analyzing BF₃**:
- In BF₃ (Boron Trifluoride), boron is bonded to three fluorine atoms. Boron has only three valence electrons and does not complete its octet, leading to a vacant p-orbital.
- This vacant p-orbital allows for back-bonding with the fluorine atoms, which creates a partial double bond character in the B-F bond. This back-bonding increases the bond strength.

3. **Analyzing CF₄**:
- In CF₄ (Carbon Tetrafluoride), carbon is bonded to four fluorine atoms. Carbon has four valence electrons and completes its octet.
- The C-F bond does not have the same back-bonding capability as BF₃ because carbon's orbitals are fully occupied. Thus, the C-F bond is a single bond without any partial double bond character.

4. **Comparing B-F and C-F Bonds**:
- The presence of back-bonding in BF₃ makes the B-F bond stronger than what would be expected from a simple single bond.
- In contrast, the C-F bond in CF₄ is a pure single bond without any additional stabilization from back-bonding.

5. **Conclusion on Assertion**:
- The assertion states that the bond dissociation energy of B-F is lower than that of C-F, which is incorrect. The B-F bond is actually stronger due to back-bonding, meaning its bond dissociation energy is higher than that of the C-F bond.

6. **Analyzing the Reason**:
- The reason states that the atomic size of B is larger than that of C. This is true; as we move from left to right in the periodic table, atomic size decreases due to increasing nuclear charge.
- Therefore, the reason provided is correct.

### Final Evaluation:
- The assertion is **false** because the bond dissociation energy of B-F is higher than that of C-F.
- The reason is **true** because the atomic size of boron is indeed larger than that of carbon.

### Answer:
- Assertion is false, but the reason is true. Thus, the correct option is **B**.