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

Ti⁺, Ti³⁺

In an aqueous solution, the lower oxidation state of Ti, which is Ti$^+$, is more stable than the higher oxidation state, Ti$^{3+}$. This is because of the inert pair effect. The inert pair effect is the tendency of the two electrons in the outermost $s$ orbital of an atom to be less chemically reactive than the electrons in the $d$ orbital. In the case of Ti, the two electrons in the $s$ orbital are more stable than the electrons in the $d$ orbital. Therefore, Ti$^+$ is more stable than Ti$^{3+}$.

The other pairs of ions in the question have a higher oxidation state being more stable. For example, Cr$^{2+}$ is more stable than Cr$^{3+}$, and Cu$^{2+}$ is more stable than Cu$^+$. This is because the higher oxidation states of these elements are more electropositive, meaning that they are more likely to lose electrons.

So the answer is (1), Ti$^+$, Ti$^{3+}$.