The correct decreasing order of spin only magnetic moment values (BM) of Cu+, Cu2+, Cr2+ and Cr3+ ions is

The spin only magnetic moment formula $\mu = \sqrt{n(n+2)}$ BM is used for transition metal ions where only electron spin contributes to magnetism. The value of $n$ (unpaired electrons) is determined from the electronic configuration using Hund’s rule. Higher the number of unpaired electrons, higher the magnetic moment. Ions with zero unpaired electrons (like $Cu^+$ with $3d^{10}$) are diamagnetic with $\mu = 0$ BM, while ions like $Cr^{2+}$ with four unpaired electrons ($3d^4$) show the highest paramagnetism among the four ions in this question.

Copper (Cu, Z=29) has the ground state configuration $[Ar]\,3d^{10}4s^1$ instead of the expected $[Ar]\,3d^9 4s^2$, because a completely filled $3d^{10}$ subshell is extra stable. Chromium (Cr, Z=24) has $[Ar]\,3d^5 4s^1$ instead of $[Ar]\,3d^4 4s^2$, because a half-filled $3d^5$ is extra stable due to maximum exchange energy and symmetrical distribution. These anomalous configurations are critical for correctly writing the configurations of their ions in JEE problems — forgetting them leads to wrong values of $n$ and wrong magnetic moments.

To write the configuration of a transition metal ion, always start from the correct ground state configuration of the neutral atom (including anomalous cases), then remove electrons from the outermost shell first. For first-row transition metals, electrons are always removed from $4s$ before $3d$. For example, $Cr^{2+}$ loses both $4s^1$ and one $3d$ electron from $[Ar]\,3d^5 4s^1$, giving $[Ar]\,3d^4$. A common mistake is writing $Cr^{2+}$ as $3d^3 4s^1$ or removing from $3d$ first — always remove $4s$ electrons first when forming cations of transition metals.

A transition metal ion is paramagnetic if it has one or more unpaired electrons, and diamagnetic if all electrons are paired. Among $d$-block ions, only those with $d^0$ or $d^{10}$ configurations are diamagnetic — examples include $Sc^{3+}$ ($d^0$), $Ti^{4+}$ ($d^0$), $Cu^+$ ($d^{10}$), $Zn^{2+}$ ($d^{10}$). All other configurations have unpaired electrons and are paramagnetic. The maximum magnetic moment in the first transition series is shown by $Mn^{2+}$ and $Fe^{3+}$, both with $d^5$ configuration giving $n=5$ and $\mu = \sqrt{35} \approx 5.92$ BM.