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 :

Which of the following lanthanoids have $[Xe] 4f^x 5d^1 6s^2$ configuration in their ground state? $(x = 1-14)$

Arrange the ions $Ce^{3+}$,$La^{3+}$,$Pm^{3+}$,and $Yb^{3+}$ in the increasing order of their ionic radii.

Compare the chemistry of the actinoids with that of lanthanoids with reference to:
$(i)$ electronic configuration
$(ii)$ oxidation states and
$(iii)$ chemical reactivity.

State the general electronic configuration of lanthanoids. Why are there irregularities in the electronic configuration of lanthanoids?

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ Actinoid contraction is greater than lanthanoid contraction.
$(b)$ Actinoids are elements of the $4f$ series.
$(c)$ All actinoid elements have a $7s^2$ configuration.