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 among the following lanthanoids forms the most stable $+2$ oxidation state?

Which of the following lanthanoids exhibits a $+4$ oxidation state with an $f^7$ configuration?

Fill in the blanks given below:
$(i)$ Neptunium and Plutonium are obtained from ............ .
$(ii)$ Lanthanoid series and Actinoid series are known as ............ .
$(iii)$ The main energy level of an atom is called ............ .
$(iv)$ Elements that possess both metallic and non-metallic properties are called ............ .

Provide the primary differences (comparison) between Lanthanoids and Actinoids.

Name the member of the lanthanoid series which is well known to exhibit $+4$ oxidation state.