Among $\mathrm{Sc^{3+}}$, $\mathrm{Ti^{4+}}$, $\mathrm{V^{2+}}$ and $\mathrm{Zn^{2+}}$, which is **coloured** in aqueous solution?

Colour in transition-metal aquo ions comes from $d$-$d$ transitions, which require partially-filled $d$-orbitals. $\mathrm{Sc^{3+}}$ ($d^0$), $\mathrm{Ti^{4+}}$ ($d^0$) and $\mathrm{Zn^{2+}}$ ($d^{10}$) all have either empty or completely filled d-shells, so they are colourless. $\mathrm{V^{2+}}$ is $d^3$, giving a violet colour in solution.