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 \(Cu =+745\,kJ\,mol ^{-1}\)

\(\Delta_{ i } H_2^{\circ} \text { of } Cu =+1960\,kJ\,mol ^{-1}\)