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

Which among the following is an actinoid element?

Which of the following oxidation states is the most common among the lanthanoids?

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

Fill in the blanks:
$1.$ The atomic radii of the elements of the third transition series are .......... to those of the corresponding elements of the second transition series.
$2.$ Samarium melts at .......... $K$.
$3.$ The color of lanthanoid ions is due to the presence of .......... electrons.

The transition element ( $\approx 5 \%$ ) present with lanthanoid metal in misch metal is