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 the assertion and the reason provided regarding the bond dissociation energies of the B-F bond in BF3 and the C-F bond in CF4, as well as the atomic sizes of boron and carbon.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the bond dissociation energy of the B-F bond in BF3 is lower than that of the C-F bond in CF4.
- To evaluate this, we need to consider the structures of both molecules and the nature of the bonds.

2. **Analyzing BF3**:
- BF3 has a trigonal planar structure with three B-F bonds.
- Boron has only three valence electrons and forms three bonds with fluorine, leading to an incomplete octet.
- Due to this incomplete octet, boron can accept lone pairs from fluorine, resulting in back bonding. This back bonding creates a partial double bond character in the B-F bond, which strengthens the bond.

3. **Analyzing CF4**:
- CF4 has a tetrahedral structure with four C-F bonds.
- Carbon has four valence electrons and forms four bonds with fluorine, achieving a complete octet.
- In CF4, there is no back bonding because carbon does not have an incomplete octet and does not accept lone pairs from fluorine. Therefore, the C-F bonds are purely single bonds.

4. **Comparing Bond Dissociation Energies**:
- The presence of back bonding in BF3 makes the B-F bond stronger than a simple single bond.
- In CF4, the C-F bond is a single bond without any additional stabilization from back bonding.
- Therefore, the bond dissociation energy of the B-F bond in BF3 is actually **greater** than that of the C-F bond in CF4, contradicting the assertion.

5. **Conclusion on the Assertion**:
- The assertion is **false** because the bond dissociation energy of the B-F bond in BF3 is greater than that of the C-F bond in CF4.

6. **Understanding the Reason**:
- The reason states that the atomic size of the B atom is larger than that of the C atom.
- This statement is **true**. As we move across a period from left to right, the atomic size decreases due to increasing nuclear charge, making boron larger than carbon.

7. **Final Evaluation**:
- The assertion is false, but the reason is true. Therefore, the correct answer is that the assertion is false while the reason is true.

### Final Answer:
- The assertion is false, and the reason is true.