${\mathrm{CuI}}_2$ is unstable even at ordinary temperature because:

Because${\mathrm{I}}^{-}$ is a big ion and the ${\mathrm{Cu}}^{2+}$ ion has a short radius and significant polarizing power, ${\mathrm{CuI}}_2$ is unstable at ambient temperature. Steric impediment is caused by its wide radius and high polarisability.
Hence, the option D is correct.