Among the following pairs of ions, the lower oxidation state in aqueous solution is more stable than the other, in:

For Thallium ($\text{Tl}$), the $+1$ oxidation state is more stable than $+3$ in aqueous solution due to the inert pair effect. Heavy group 13 elements prefer the lower oxidation state because the $s$ -electrons are strongly held by the nucleus. In other options, the higher state (${\text{Cu}}^{2+}$, ${\text{Cr}}^{3+}$) is generally more stable in water.