d
The stability of $Cu ^{2 +}( aq )$ is more than $Cu ^+ ( aq )$ is due to the much more negative $\Delta_{\text {hyd }} H^{\circ}$ of $Cu ^{2+}( aq )$ than $Cu ^{+}( aq )$, which more than compensates for second ionisation enthalpy of $Cu$.
$\Delta \text { hyd } H ^{\circ} \text { of } Cu ^{2+}( aq )=-2121\,kJ\,mol ^{-1}$
$\Delta_{ i } H _1^0$ of $Cu =+745\,kJ\,mol ^{-1}$
$\Delta_{ i } H_2^{\circ} \text { of } Cu =+1960\,kJ\,mol ^{-1}$