Which of the following compounds is the most acidic?

Mischmetal is an alloy consisting mainly of

Most common oxidation states shown by cerium are

Although $(+3)$ oxidation state is the characteristic oxidation state of lanthanoids,cerium shows $(+4)$ oxidation state also. Why?

Which of the following elements is not an actinoid?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Size of $Bk^{3+}$ ion is less than $Np^{3+}$ ion.
Reason $R$: The above is a consequence of the lanthanoid contraction.
In the light of the above statements,choose the correct answer from the options given below: