Which one of the following does not correctly represent the correct order of the property indicated against it

Options

A. `Ti^(3+) lt V^(3+) lt Cr^(3+) lt Mn^(3+)`: increasing magnetic moment
B. `Ti lt V lt Cr lt Mn` : increasing melting point
C. `Ti lt V lt Mn lt Cr` : increasing `2^(nd)` ionisation enthalpy
D. `Ti lt V lt Cr lt Mn` : increasing number of oxidation states

Explanation

To solve the question regarding the correct order of properties for the given transition metal ions, we will analyze each option step by step.

### Step 1: Analyze the first option - Increasing magnetic moment
The magnetic moment can be calculated using the number of unpaired electrons in the d-orbitals of the transition metal ions.

- **Ti³⁺ (Titanium)**: Configuration is [Ar] 3d¹. It has 1 unpaired electron.
- **V³⁺ (Vanadium)**: Configuration is [Ar] 3d². It has 2 unpaired electrons.
- **Cr³⁺ (Chromium)**: Configuration is [Ar] 3d³. It has 3 unpaired electrons.
- **Mn³⁺ (Manganese)**: Configuration is [Ar] 3d⁴. It has 4 unpaired electrons.

The order of increasing magnetic moment based on the number of unpaired electrons is:
**Ti³⁺ < V³⁺ < Cr³⁺ < Mn³⁺** (This order is correct).

### Step 2: Analyze the second option - Increasing melting point
The melting points of the transition metals generally increase across the series due to increasing metallic bonding.

- **Ti**: Melting point is lower than that of V.
- **V**: Has a high melting point.
- **Cr**: Has a very high melting point, slightly lower than V.
- **Mn**: Has a lower melting point compared to V and Cr.

The correct order of melting points is:
**Ti < Mn < Cr < V** (This order is incorrect as it suggests Mn has a higher melting point than Cr).

### Step 3: Analyze the third option - Increasing second ionization enthalpy
The second ionization enthalpy generally increases across the transition series due to increased nuclear charge and decreased atomic radius.

- **Ti**: 1320 kJ/mol
- **V**: 1376 kJ/mol
- **Cr**: 1635 kJ/mol
- **Mn**: 1513 kJ/mol

The order of increasing second ionization enthalpy is:
**Ti < V < Mn < Cr** (This order is correct).

### Step 4: Analyze the fourth option - Increasing number of oxidation states
The number of oxidation states generally increases from Scandium to Manganese and then decreases towards Zinc.

- **Sc**: +3
- **Ti**: +4
- **V**: +5
- **Cr**: +6
- **Mn**: +7

Thus, the increasing order of oxidation states is:
**Ti < V < Cr < Mn** (This order is correct).

### Conclusion
The only incorrect order is found in the second option regarding melting points. Therefore, the answer to the question is:

**Answer: B (Ti < V < Mn < Cr for melting point is incorrect)**.