Which of the following ions is diamagnetic? (Th: $6d^{2}7s^2$, U: $5f^{3}6d^{1}7s^2$, Np: $5f^{4}6d^{1}7s^2$, Pu: $5f^{6}7s^2$)

$\mathrm{Th}{}^{4+}$ has lost all valence electrons and has the radon core (no unpaired electron). $\mathrm{U}{}^{3+}$ is $5f^3$, $\mathrm{Np}{}^{3+}$ is $5f^4$ and $\mathrm{Pu}{}^{3+}$ is $5f^5$, all with unpaired electrons and paramagnetic.