Assertion (A) : When the transition element ionises, the `4s`-orbital electrons are removed before the `3d`-orbital electrons.
Reason (R ) : The energy of `3d`-orbital electrons is lower than that of `4s`-orbital electrons.

Options

A. If both Assertion (A) and (R ) are correct and Reason (R ) is the correct explanation of Assertion (A).
B. If both (A) and (R ) are correct but (R ) is not the correct explanation for (A).
C. If (A) is correct but (R ) is not correct.
D. If (A) and (R ) are correct

Answer

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

### Step 1: Understanding the Assertion (A)
The assertion states that when transition elements ionize, the 4s orbital electrons are removed before the 3d electrons.

- **Explanation**: In transition metals, when they lose electrons during ionization, the 4s electrons are indeed removed before the 3d electrons. This is because, although the 4s orbital is filled before the 3d orbital during the building-up process (Aufbau principle), the 4s electrons are at a higher energy level when the atom is in an ionized state.

### Step 2: Understanding the Reason (R)
The reason states that the energy of 3d orbital electrons is lower than that of 4s orbital electrons.

- **Explanation**: This statement is incorrect. In the context of transition metals, the energy of the 3d electrons is actually higher than that of the 4s electrons when considering the ionized state. The 4s electrons are removed first because they are at a higher energy level in the ionized state.

### Step 3: Conclusion
- The assertion (A) is true: 4s electrons are removed before 3d electrons during ionization of transition elements.
- The reason (R) is false: The energy of 3d electrons is not lower than that of 4s electrons; it is actually higher.

### Final Answer
- **Assertion (A)** is correct.
- **Reason (R)** is incorrect.
- Therefore, the correct answer is that A is true, but R is false.