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Which ion is colourless :
A
$Cr^{4+}$
B
$Sc^{3+}$
C
$Ti^{3+}$
D
$V^{3+}$
Detailed Solution
The colour of transition metal ions arises from d–d transitions, which need partly filled d orbitals.
$Sc^{3+}$: $3d^0$ (no d electrons), so no d–d transition is possible, and it is colourless.
$Ti^{3+}$ ($3d^1$, purple), $V^{3+}$ ($3d^2$, green) and $Cr^{4+}$ ($3d^2$) have unpaired d electrons and are coloured.
So $Sc^{3+}$ is colourless.
$Sc^{3+}$: $3d^0$ (no d electrons), so no d–d transition is possible, and it is colourless.
$Ti^{3+}$ ($3d^1$, purple), $V^{3+}$ ($3d^2$, green) and $Cr^{4+}$ ($3d^2$) have unpaired d electrons and are coloured.
So $Sc^{3+}$ is colourless.