What is the composition of metals in pyrophoric mischmetal?

The lanthanide ion having four unpaired electrons is
(Given : Atomic number of $Ce = 58, Nd = 60, Tb = 65$ and $Ho = 67$)

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 actinoids exhibit a greater number of oxidation states in general than the lanthanoids. This is because

Which among the following elements is radioactive?

Given below are two statements:
Statement $I$: In the Lanthanoids,the formation of $Ce^{+4}$ is favoured by its noble gas configuration.
Statement $II$: $Ce^{+4}$ is a strong oxidant reverting to the common $+3$ state.
In the light of the above statements,choose the most appropriate answer from the options given below: