Assertion: La should be the f-block element according to Aufbau principle.
Reason: 57th electron exceptionally enters in 5d orbital.
If both (A) and (R) are correct and (R) is the correct explanation of (A).
If both (A) and (R) are correct, but (R) is not the correct explanation of (A).
If (A) is correct, but (R) is incorrect.
If (A) is incorrect but (R) is correct.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct, but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect but (R) is correct.

Explanation

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided in the question.

### Step 1: Understanding the Assertion
The assertion states that "La should be the f-block element according to the Aufbau principle." According to the Aufbau principle, electrons fill orbitals in order of increasing energy. Lanthanum (La) has the atomic number 57, and its expected electron configuration would be based on filling the 4f and 6s orbitals.

### Step 2: Writing the Expected Electron Configuration
If we follow the Aufbau principle strictly, the expected electron configuration for Lanthanum would be:
- \( \text{Xe} \, 4f^1 \, 6s^2 \)

This configuration suggests that Lanthanum should indeed be classified as an f-block element since it has an electron in the 4f orbital.

### Step 3: Analyzing the Actual Electron Configuration
However, the actual electron configuration of Lanthanum is:
- \( \text{Xe} \, 5d^1 \, 6s^2 \)

This indicates that the 57th electron enters the 5d orbital instead of the 4f orbital.

### Step 4: Understanding the Reason
The reason states that "the 57th electron exceptionally enters the 5d orbital." This is due to the fact that the energies of the 4f and 5d orbitals are very close, leading to the 5d orbital being filled first in this case.

### Step 5: Evaluating the Assertion and Reason
- **Assertion (A)**: True, because according to the Aufbau principle, Lanthanum could be considered an f-block element based on its expected configuration.
- **Reason (R)**: True, because the 57th electron does indeed enter the 5d orbital instead of the 4f orbital due to their similar energy levels.

### Step 6: Determining the Relationship
While both the assertion and reason are true, the reason does not correctly explain why Lanthanum is classified as a d-block element instead of an f-block element. The reason is more about the behavior of the electron filling rather than justifying the classification.

### Conclusion
The correct answer is:
**If both (A) and (R) are correct, but (R) is not the correct explanation of (A).**

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