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Out of $TiF_6^{2-}$, $CoF_6^{3-}$, $Cu_2Cl_2$ and $NiCl_4^{2-}$ (Z of Ti = 22, Co = 27, Cu = 29, Ni = 28) the colourless species are
A
$CoF_6^{3-}$ and $NiCl_4^{2-}$
B
$TiF_6^{2-}$ and $CoF_6^{3-}$
C
$Cu_2Cl_2$ and $NiCl_4^{2-}$
D
$TiF_6^{2-}$ and $Cu_2Cl_2$
Detailed Solution
Transition metal species are coloured when they have unpaired d electrons that can undergo d–d transitions; species with $d^0$ or $d^{10}$ configuration are colourless.
$TiF_6^{2-}$: Ti is in the +4 state, $3d^0$; colourless.
$CoF_6^{3-}$: Co is in the +3 state, $3d^6$ with 4 unpaired electrons (weak field $F^-$); coloured.
$Cu_2Cl_2$: Cu is in the +1 state, $3d^{10}$; colourless.
$NiCl_4^{2-}$: Ni is in the +2 state, $3d^8$ with 2 unpaired electrons; coloured.
Hence the colourless species are $TiF_6^{2-}$ and $Cu_2Cl_2$.
$TiF_6^{2-}$: Ti is in the +4 state, $3d^0$; colourless.
$CoF_6^{3-}$: Co is in the +3 state, $3d^6$ with 4 unpaired electrons (weak field $F^-$); coloured.
$Cu_2Cl_2$: Cu is in the +1 state, $3d^{10}$; colourless.
$NiCl_4^{2-}$: Ni is in the +2 state, $3d^8$ with 2 unpaired electrons; coloured.
Hence the colourless species are $TiF_6^{2-}$ and $Cu_2Cl_2$.