Among `TiF_(6)^(2-), CoF_(6)^(3-), Cu_(2)C1_(2)` and `NiC1_(4)^(2-)` (At. No. `Ti = 22, Co = 27, Cu = 29, Ni = 28)`, the colourless species are -

Explanation

In `[TiF_(6)]^(2-)` the titanium is in `+4` oxidation state having the electronic configuration `[Ar]^(18)3d^(0)4s^(0)`. Similarly in `Cu_(2)Cl_(2)` the copper is in `+1` oxidation state having the electronic configuration `[Ar]^(18)3d^(10)4s^(0)`. As they do not have any unpaired electrons for d- d transition, they are therefore colourless.
In `[NiCl_(4)]^(2-)` the nickel is in `+2` oxidation state and electronic configuration is `[Ar]^(18)3d^(8)4s^(0)`. As it has two unpaired electrons, so the complex is coloured. In `[CoF_(6)]^(3-)`, the cobalt is in `+3` oxidation state having electron configuration `[Ar]3d^(6)4s^(0)`. As it has four unpaired electron, so the complex is coloured.