(A): Zr and Hf have nearly equal atomic radii.
(R): Zr and Hf belong to the same group.

Options

A. Both A & R are true, R is the correct explanation of A
B. Both A & R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Answer

To analyze the given question, we need to evaluate both the assertion (A) and the reason (R) provided:

**Assertion (A):** Zr and Hf have nearly equal atomic radii.
**Reason (R):** Zr and Hf belong to the same group.

### Step-by-Step Solution:

1. **Identify the Elements:**
- Zr stands for Zirconium, which is in the 4th period and belongs to the 4d series.
- Hf stands for Hafnium, which is in the 5th period and belongs to the 5d series.

2. **Understanding Atomic Radii:**
- Atomic radius generally refers to the size of an atom, which can be influenced by various factors including the number of electron shells and the effective nuclear charge experienced by the outermost electrons.

3. **Comparing Atomic Radii of Zr and Hf:**
- Despite being in different periods (4d for Zr and 5d for Hf), Zr and Hf have nearly equal atomic radii. This is an important observation that needs to be explained.

4. **Role of Shielding Effect:**
- The shielding effect refers to the phenomenon where inner shell electrons reduce the effective nuclear charge felt by the outer shell electrons. This effect can lead to similar atomic sizes in elements from different periods if the shielding is comparable.
- In the case of Zr and Hf, the presence of d-electrons in both elements leads to similar shielding effects, resulting in nearly equal atomic radii.

5. **Evaluating the Reason:**
- The reason states that Zr and Hf belong to the same group. While this is true (as they are both in Group 4), it does not adequately explain why their atomic radii are nearly equal.
- The assertion is true, but the reason does not correctly justify the assertion.

6. **Conclusion:**
- Both the assertion and reason are true statements, but the reason does not provide a correct explanation for the assertion. Therefore, the correct answer is that the assertion is true, but the reason is not the correct explanation.

### Final Answer:
- The assertion (A) is true, and the reason (R) is true but does not correctly explain the assertion. Thus, the answer is option number two.

---