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

We analyze each order based on NCERT data:

1. Magnetic Moment: The number of unpaired electrons ($n$) increases from $Ti^{3+}(d^1, n=1)$ to $Mn^{3+}(d^4, n=4)$. Since magnetic moment $\mu = \sqrt{n(n+2)}$, the order is correct .
2. Melting Point: The melting points of transition metals generally rise to a maximum near the middle of the series ($Cr$). However, Manganese ($Mn$) shows an anomalous low melting point ($1246 \text{ K}$) compared to Chromium ($2180 \text{ K}$) and Vanadium ($1910 \text{ K}$), due to its stable $d^5$ configuration leading to weaker metallic bonding. Thus, the order $Ti < V < Cr < Mn$ is incorrect because Mn is lower than V and Cr .
3. 2nd Ionization Enthalpy: The second ionization enthalpy involves removing an electron from the singly charged ion. For $Cr^+ (3d^5)$, removing an electron disrupts the stable half-filled configuration, leading to a very high value ($1592 \text{ kJ mol}^{-1}$). For $Mn^+ (3d^5 4s^1)$, the electron is removed from $4s$, leaving the stable $3d^5$ intact ($1509 \text{ kJ mol}^{-1}$). Thus, $Cr > Mn$, making the order $Ti < V < Mn < Cr$ correct .
4. Oxidation States: The number of oxidation states increases from Ti (+2 to +4) to Mn (+2 to +7). The order is correct .