Assertion: The second ionisation energies of V, Cr and Mn are in the order `VltCrltMn`.
Reason: Ionisation energies show a regular increase along a period with increase of atomic number.

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 regarding the assertion and reason about the second ionization energies of vanadium (V), chromium (Cr), and manganese (Mn), we will analyze both statements step by step.

### Step 1: Understanding the Assertion
The assertion states that the second ionization energies of V, Cr, and Mn are in the order:
\[ \text{V} < \text{Cr} < \text{Mn} \]

**Analysis**:
- The second ionization energy refers to the energy required to remove the second electron from an atom after the first electron has already been removed.
- For vanadium (V), after the first ionization, the electron configuration becomes \( \text{[Ar]} 3d^3 4s^1 \). Removing a second electron from this configuration will require less energy because it will lead to a more stable configuration.
- For chromium (Cr), the configuration after the first ionization becomes \( \text{[Ar]} 3d^5 \). This half-filled d-subshell is stable, and removing a second electron from this stable configuration requires more energy.
- For manganese (Mn), the configuration after the first ionization is \( \text{[Ar]} 3d^5 4s^1 \). The removal of the second electron will also lead to a stable half-filled d-subshell, but the effective nuclear charge is higher than that of vanadium, which means it will require more energy compared to vanadium.

### Conclusion for Step 1:
The assertion that the second ionization energies are in the order \( \text{V} < \text{Cr} < \text{Mn} \) is **incorrect**. The correct order should be \( \text{Cr} > \text{Mn} > \text{V} \).

### Step 2: Understanding the Reason
The reason states that "Ionization energies show a regular increase along a period with an increase in atomic number."

**Analysis**:
- This statement is generally true. As we move across a period from left to right, the atomic number increases, and electrons are added to the same shell. This leads to an increase in effective nuclear charge, making it harder to remove electrons.
- Therefore, the ionization energy tends to increase as we move across a period.

### Conclusion for Step 2:
The reason provided is **correct**.

### Final Conclusion:
- The assertion is **false**.
- The reason is **true**.
- Therefore, the correct answer is that the assertion is incorrect while the reason is correct.

### Answer:
The correct option is **Option 4**: Assertion is incorrect, but reason is correct.

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