Write the electron configurations of V²⁺, Co³⁺, Cu⁺ and Mn²⁺.

Step 1. $\text{V}^{2+}$. $[\text{Ar}]\,3d^3$: $3$ unpaired, $\mu = \sqrt{15} = 3.87\ \text{BM}$.

Step 2. $\text{Co}^{3+}$. $[\text{Ar}]\,3d^6$: $4$ unpaired, $\mu = \sqrt{24} = 4.90\ \text{BM}$.

Step 3. $\text{Cu}^+$. $[\text{Ar}]\,3d^{10}$: none unpaired, $\mu = 0$.

Step 4. $\text{Mn}^{2+}$. $[\text{Ar}]\,3d^5$: $5$ unpaired, $\mu = \sqrt{35} = 5.92\ \text{BM}$.