Identify the lanthanoid that exhibits zero effective magnetic moment in $+3$ state.

Consider the following two statements:
$Statement-I:$ Among $Mg, Al, Mg^{2+}, Al^{3+}$,the one having the smallest size is $Al^{3+}$.
$Statement-II:$ $Eu$ has an exceptionally high atomic radius among lanthanide elements.
The correct answer is:

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ All actinoid elements have a $7s^1$ configuration.
$(b)$ All actinoid elements have a $7s^0$ configuration.
$(c)$ There are no electrons in the $7s$ orbital of actinoids.

Number of colourless lanthanoid ions among the following is . . . . .
$Eu^{3+}, Lu^{3+}, Nd^{3+}, La^{3+}, Sm^{3+}$

Lanthanoid ions with $4f^7$ configuration are :
$A$. $Eu^{2+}$
$B$. $Gd^{3+}$
$C$. $Eu^{3+}$
$D$. $Tb^{3+}$
$E$. $Sm^{2+}$
Choose the correct answer from the options given below :

Identify the isoelectronic pair of ions from the following.