The most basic oxide of lanthanoids is $:-$

Which one amongst the following are good oxidizing agents?
$A.$ $Sm^{2+}$
$B.$ $Ce^{2+}$
$C.$ $Ce^{4+}$
$D.$ $Tb^{4+}$
Choose the most appropriate answer from the options given below:

Choose the correct option regarding the following.
Assertion: Actinides exhibit a larger number of oxidation states than lanthanides.
Reason: There is a large energy gap between $5f$,$6d$ and $7s$ sub-shells.

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 :

The atomic size decreases with an increase in atomic number for the elements of the ........

Identify the lanthanoid element from the following.