Which oxidation state is common for lanthanoid elements?

Which one amongst the following are good oxidizing agents?
$A.$ $Sm^{2+}$
$B.$ $Ce^{2+}$
$C.$ $Ce^{4+}$
$D.$ $Tb^{4+}$
Choose the most appropriate answer from the options given below:

The total number of rare-earth elements is

Determine the correct $(T)$ and incorrect $(F)$ statements from the following:
$(a)$ The symbol for Lanthanum is $La$.
$(b)$ The name of the element $Lu$ is Lanthanum.
$(c)$ The electronic configuration of $La^{3+}$ and $Ce^{4+}$ contains $4f^0$.

Lanthanide contraction occurs because

Mischmetal is an alloy consisting mainly of