NEET 2023, Chemistry Q28: Colligative Properties

Going Cu $\to$ Cu${}^{+}$ $\to$ Cu${}^{2+}$ costs additional ionisation energy, but in aqueous solution the much larger hydration enthalpy released by the smaller, doubly-charged Cu${}^{2+}$ ion more than compensates for the higher IE${}_2$.

Net result: Cu${}^{2+}$ is the thermodynamically stable species in water; Cu${}^{+}$ disproportionates to Cu and Cu${}^{2+}$.