Amongst `[TiE_(6)]^(2-), [CoF_(6)]^(3-), Cu_(2)Cl_(2)` and `[NiCl_(4)]^(2-)` [Atomic no. `Ti = 22, Co = 27, Cu = 29, Ni = 28`] the colourless species are :
(A) `[TiF_(6)]^(2-)` and `[Cu_(2)Cl_(2)]`
(B) `Cu_(2)Cl_(2)` and `[NiCl_(4)]^(2-)`
(C) `[TiF_(6)]^(2-)` and `[CoF_(6)]^(3-)`
(D) `[CoF_(6)]^(3-)` and `[NiCl_(4)]^(2-)`

Explanation

(A). In `[TiF_(6)]^(2-)` the titanium is in +4 oxidation state having electronic configuration `[Ar]^(18)3d^(0)4s^(0)` similarly in `Cu_(2)Cl_(2)` The copper is in +1 oxidation state having the electronic configuraiton `[Ar]^(18)d^(10)4s^(0)`
As they do not have any unpaired electrons for d-d transiton, they are thereofre colourless.
In `[NiCl_(4)]^(2-)` the nickel is in +2 oxidation state and electroniconfiguration 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 electrons, so the complex is coloured.