Which one of the following ions is the most stable in aqueous...

In aqueous solution the configuration of the ions are following $$\begin{aligned} \mathrm{Cr}^{3+}(21) & =3 d^3 4 s^0 \Rightarrow t_{2 g}{ }^3 e_g^0 \\ \mathrm{ V}^{3+}(20) & =3 d^2 4 s^0 \Rightarrow t_{28}{ }^2 e_g^0 \\ \mathrm{Tr}^{3+}(19) & =3 d^1 4 s^0 \Rightarrow t_{28}{ }^1 e_g^0 \\ \mathrm{Mn}^{3+}(22) & =3 d^4 4 s^0 \Rightarrow t_{2 g}{ }^4 e_g^0 \end{aligned}$$ We know that half-filled and fully filled orbitals are more stable than partially filled orbitals. Hence among the given ions the only $\mathrm{Cr}^{3+}$ ion which occupies half-filled $t_{2 g}$ orbitals. Caution Although $\mathrm{Mn}^{3+}$ ion has half-filled $t_{\text{s orbital }}$ but there is one more electron filled in $e_R$ orbital which reduces the stability.