The ions from among the following which are colourless are:

(i) ${\mathrm{Ti}}^{4+}$   (ii) ${\mathrm{Cu}}^{+}$  (iii) ${\mathrm{Co}}^{3+}$    (iv) ${\mathrm{Fe}}^{2+}$

(i) Valence shell electron configuration of ${\mathrm{Ti}}^{4+} \mathrm{is} 3{\mathrm{d}}^{0}4{\mathrm{s}}^0$. As there is no unpaired electrons for d-d transition, the solution of ions will be colourless.
(ii) Valence shell electron configuration of ${\mathrm{Cu}}^{+}\text{\hspace{0.33em}is\hspace{0.33em}}3{\mathrm{d}}^{10}4{\mathrm{s}}^0$. As all electrons are paired, there is no d-d transition, so the solution of ions will be colourless.
(iii) Valence shell electron configuration of ${\mathrm{Co}}^{3+}\text{\hspace{0.33em}is\hspace{0.33em}}3{\mathrm{d}}^{6}4{\mathrm{s}}^0$. As there are 4 unpaired electrons, there is d-d transition of electron, so the solution of ions will be coloured.
(iv) Valence shell electron configuration of ${\mathrm{Fe}}^{2+}\text{\hspace{0.33em}is\hspace{0.33em}}3{\mathrm{d}}^{6}4{\mathrm{s}}^0$. As there are 4 unpaired electrons, there is d-d transition of electron, so the solution of ions will be coloured.