Assertion:Boron can only form `[BF_(4)]^(-)`,whereas aluminium forms`[AlF_(6)]^(3-)`.
Reason:Ionization enthalpy increases across a period.

Options

A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that boron can only form \([BF_4]^-\) while aluminum can form \([AlF_6]^{3-}\).
- Boron (atomic number 5) has the electronic configuration of \(1s^2 2s^2 2p^1\). It has a total of four valence electrons (2 from \(2s\) and 1 from \(2p\)).
- Due to its small size and limited number of available orbitals (only \(s\) and \(p\)), boron can only expand its valency up to 4. Thus, it can form a maximum of four bonds, leading to the formation of \([BF_4]^-\).
- Aluminum (atomic number 13) has the electronic configuration of \(Ne: 3s^2 3p^1\) and also has vacant \(d\) orbitals. This allows aluminum to expand its valency beyond 4 and form \([AlF_6]^{3-}\).

2. **Understanding the Reason**:
- The reason states that ionization enthalpy increases across a period.
- Ionization enthalpy is the energy required to remove an electron from an isolated atom. As we move across a period, the atomic size decreases due to increased effective nuclear charge, making it more difficult to remove electrons. Therefore, the ionization enthalpy increases across a period.

3. **Analyzing the Relationship**:
- Both the assertion and the reason are correct statements.
- However, the reason provided (ionization enthalpy increasing across a period) does not directly explain why boron can only form \([BF_4]^-\) and aluminum can form \([AlF_6]^{3-}\). The assertion is based on the electronic configuration and the ability to expand valency, while the reason discusses a general trend in ionization enthalpy.

4. **Conclusion**:
- Both the assertion and the reason are correct, but the reason is not the correct explanation for the assertion.
- Therefore, the correct answer is that both the assertion and reason are correct, but the reason is not the correct explanation for the assertion.

### Final Answer:
Both assertion and reason are correct, but the reason is not the correct explanation for the assertion.