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The pair of lanthanoid ions which are diamagnetic is
A
$Gd^{3+}$ and $Eu^{3+}$
B
$Pm^{3+}$ and $Sm^{3+}$
C
$Ce^{4+}$ and $Yb^{2+}$
D
$Ce^{3+}$ and $Eu^{2+}$
Detailed Solution
$Ce^{4+} = [Xe]\,4f^0$: no unpaired electrons
$Yb^{2+} = [Xe]\,4f^{14}$: all electrons paired
So $Ce^{4+}$ and $Yb^{2+}$ are both diamagnetic.