Which lanthanoid from the following may exhibit $+4$ oxidation state with $f^{0}$ configuration?

Which of the following elements have half-filled $f$-orbitals in their ground state?
(Given: atomic number $Sm = 62$; $Eu = 63$; $Tb = 65$; $Gd = 64$; $Pm = 61$)
$A.$ $Sm$ $B.$ $Eu$ $C.$ $Tb$ $D.$ $Gd$ $E.$ $Pm$
Choose the correct answer from the options given below:

Identify the factor responsible for the greater range of oxidation states in actinoids.

Most basic hydroxide among the following is

What is the composition of metals in pyrophoric mischmetal?

Assertion : Europium $(II)$ is more stable than cerium $(II)$.
Reason : Cerium salts are used as catalyst in petroleum cracking.