Consider the transition metal ions Mn^ 3+ , Cr^ 3+ , Fe^ 3+ and Co^...

For low spin octahedral complexes, electrons fill the $t_{2g}$ orbitals first before pairing or moving to $e_g$ orbitals. The configuration depends on the number of d-electrons.
For $Cr^{3+}$: Electronic configuration is $3d^3$. In an octahedral field, the configuration is $t_{2g}^3 e_g^0$. Number of unpaired electrons ($n$) = 3.
For $Mn^{3+}$: Electronic configuration is $3d^4$. For low spin, the configuration is $t_{2g}^4 e_g^0$. Number of unpaired electrons ($n$) = 2.
For $Fe^{3+}$: Electronic configuration is $3d^5$. For low spin, the configuration is $t_{2g}^5 e_g^0$. Number of unpaired electrons ($n$) = 1.
For $Co^{3+}$: Electronic configuration is $3d^6$. For low spin, the configuration is $t_{2g}^6 e_g^0$. Number of unpaired electrons ($n$) = 0.
Comparing the number of unpaired electrons:
$Cr^{3+} (3) > Mn^{3+} (2) > Fe^{3+} (1) > Co^{3+} (0)$.
The correct decreasing order is $Cr^{3+} > Mn^{3+} > Fe^{3+} > Co^{3+}$.