Which one of the following is a radioactive lanthanide?

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 ............ .

Identify the element with the smallest ionic radius in the $+3$ oxidation state from the following:

The electronic configurations of $Eu$ (Atomic No. $63$),$Gd$ (Atomic No. $64$) and $Tb$ (Atomic No. $65$) are

Although $(+3)$ oxidation state is the characteristic oxidation state of lanthanoids,cerium shows $(+4)$ oxidation state also. Why?

Which of the following Lanthanoid ions are diamagnetic?