In which of the following pairs are both the ions coloured in aqueous solution?(At. no. : Sc = 21, Ti = 22, Ni = 28, Cu = 29, Co= 27)

The colour of transition metal ions in aqueous solution is due to d-d transitions, which require unpaired d-electrons. ${\text{Ni}}^{2+}$ ($3d^8$, 2 unpaired electrons) and ${\text{Ti}}^{3+}$ ($3d^1$, 1 unpaired electron) both have unpaired electrons and are coloured. ${\text{Sc}}^{3+}$ ($3d^0$) and ${\text{Cu}}^{+}$ ($3d^{10}$) have 0 unpaired electrons and are colourless.