The pair of lanthanoid ions which are diamagnetic is:

Diamagnetism requires the absence of unpaired electrons.${\text{Ce}}^{4+}$: $\text{Ce}(Z=58)$ is $[\text{Xe}]4f^{1}5d^{1}6s^2$. ${\text{Ce}}^{4+}$ is $[\text{Xe}]4f^0$. No unpaired electrons.${\text{Yb}}^{2+}$: $\text{Yb}(Z=70)$ is $[\text{Xe}]4f^{14}6s^2$. ${\text{Yb}}^{2+}$ is $[\text{Xe}]4f^{14}$. Fully filled $f$ -subshell, no unpaired electrons.Both are diamagnetic.