Assertion : While filling different subshells in an atom, electrons enter into 3d subshell before filling 4p subshell.
Reason : Electrons prefer to remain unpaired as far as possible.

Options

A. If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
B. If both Assertion and Reason are correct and Reason is not the correct explanation of Assertion.
C. If Assertion is correct but Reason is incorrect.
D. If Assertion is incorrect but Reason is correct.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that while filling different subshells in an atom, electrons enter the 3d subshell before filling the 4p subshell.
- According to the Aufbau principle, electrons are filled into atomic orbitals in order of increasing energy levels.

2. **Energy Levels of Subshells**:
- The energy levels of subshells can be compared as follows:
- 3s < 3p < 4s < 3d < 4p < 4d < 5s
- From this order, we see that the 3d subshell has a lower energy than the 4p subshell. Therefore, electrons will fill the 3d subshell before the 4p subshell.

3. **Conclusion about the Assertion**:
- Since electrons indeed fill the 3d subshell before the 4p subshell based on energy levels, the assertion is **true**.

4. **Understanding the Reason**:
- The reason states that electrons prefer to remain unpaired as far as possible.
- While it is true that electrons tend to occupy degenerate orbitals singly before pairing (Hund's rule), this is not the reason for the filling order of the 3d and 4p subshells.

5. **Conclusion about the Reason**:
- The reason provided does not accurately explain the filling order of the subshells. The filling order is determined by energy levels, not by the preference of electrons to remain unpaired. Therefore, the reason is **false**.

6. **Final Assessment**:
- The assertion is true, but the reason is false. Hence, the correct option for this question is that the assertion is correct, but the reason is incorrect.

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