The ions from among the following which are colourless are:(i) Ti4+   (ii) Cu+  (iii) Co3+    (iv) Fe2+

(i) Valence shell electron configuration of Ti4+ is 3d04s0. As there is no unpaired electrons for d-d transition, the solution of ions will be colourless.(ii) Valence shell electron configuration of Cu+ is 3d104s0. 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 Co3+ is 3d64s0. 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 Fe2+ is 3d64s0. As there are 4 unpaired electrons, there is d-d transition of electron, so the solution of ions will be coloured.