Among the following aqueous tripositive ions, the one that is colourless is

Colour of lanthanoid ions arises from $f$-$f$ transitions, which need partly filled $4f$ subshell. $\mathrm{La}{}^{3+}$ is $4f^0$, so it has no such transition and is colourless, whereas $\mathrm{Pr}{}^{3+}$ ($4f^2$), $\mathrm{Nd}{}^{3+}$ ($4f^3$) and $\mathrm{Sm}{}^{3+}$ ($4f^5$) are coloured.