Which of the following elements are included in the actinoid series?

Explain lanthanoid contraction in detail. What is lanthanoid contraction? What are its consequences?

Which of the following elements can form compounds in $+3$ to $+7$ oxidation states?

Observe the following $f$-block elements: $Eu (Z=63)$; $Pu (Z=94)$; $Cf (Z=98)$; $Sm (Z=62)$; $Gd (Z=64)$; $Cm (Z=96)$. How many of the above have half-filled $f$-orbitals in their ground 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.

Which element from the following has a completely filled $f$-orbital in both observed and expected electronic configurations?