(a). Assertion (A) is true, Reason (R) is also true, Reason (R) is the correct explanation for assertion (A).
(b). Assertion (A) is true, Reason (R) is true, Reason (R) is not the correct explanation for Assertion (A).
(c). Assertion (A) is true, Reason (R) is false.
(d). Assertion (A) is false, Reason (R) is true.
Q. Assertion (A): `Zn^(2+)` is diamagnetic.
Reason (R): The electrons are lost from 4s-orbital to form `Zn^(2+)`.

Explanation

To solve the assertion and reason question regarding the magnetic properties of \( \text{Zn}^{2+} \) and the electron configuration, we will analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Analyze the Assertion (A)
**Assertion (A):** \( \text{Zn}^{2+} \) is diamagnetic.

- **Explanation:** Diamagnetism occurs when all the electrons in an atom or ion are paired. In the case of \( \text{Zn}^{2+} \), we need to look at its electron configuration. The atomic number of zinc (Zn) is 30, and its electron configuration is:
\[ \text{Zn: } [\text{Ar}] 4s^2 3d^{10} \]
When zinc loses two electrons to form \( \text{Zn}^{2+} \), the electrons are removed from the 4s orbital:
\[ \text{Zn}^{2+}: [\text{Ar}] 3d^{10} \]
Since all the electrons in \( 3d^{10} \) are paired, \( \text{Zn}^{2+} \) is indeed diamagnetic.

### Step 2: Analyze the Reason (R)
**Reason (R):** The electrons are lost from the 4s-orbital to form \( \text{Zn}^{2+} \).

- **Explanation:** This statement is true. When zinc forms \( \text{Zn}^{2+} \), the two electrons from the 4s orbital are lost first before any 3d electrons. This is consistent with the general trend observed in transition metals where the 4s electrons are removed before the 3d electrons.

### Step 3: Determine the Relationship Between Assertion and Reason
Now that we have established that both the assertion and the reason are true, we need to evaluate whether the reason is the correct explanation for the assertion.

- The assertion states that \( \text{Zn}^{2+} \) is diamagnetic due to the absence of unpaired electrons. The reason states that the electrons are lost from the 4s orbital. While both statements are true, the reason does not directly explain why \( \text{Zn}^{2+} \) is diamagnetic; it merely describes the process of how \( \text{Zn}^{2+} \) is formed.

### Conclusion
Based on the analysis:
- Assertion (A) is true.
- Reason (R) is true.
- However, Reason (R) is not the correct explanation for Assertion (A).

Thus, the correct answer is:
**(b). Assertion (A) is true, Reason (R) is true, Reason (R) is not the correct explanation for Assertion (A).**