JEE Main · 2005 · 08 May 2005 · Offline · d and f Block Elements

A spin-only moment of $2.84$ BM points, through $\mu =\sqrt{n(n+2)}$, to $n=2$ unpaired electrons, since $\sqrt{2\times 4}=\sqrt{8}\approx 2.83$. A $d^4$ ion in a strong field pairs electrons into the lower $t_{2g}$ set, reaching $t_{2g}^4{e}_g^0$ with two of the four electrons paired and two left unpaired, which gives exactly $n=2$. Weak-field $d^4$ would keep all four unpaired and $d^3$ always keeps three, so the match is $d^4$ in a strong field.