Which of the following elements belongs to the actinide series?

Lanthanide contraction occurs because

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

Name the members of the lanthanoid series which exhibit $+4$ oxidation state and those which exhibit $+2$ oxidation state. Try to correlate this type of behavior with the electronic configurations of these elements.

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

Which one of the lanthanoids given below is the most stable in divalent form?