From the list given below,the number of lanthanides which exhibit $+4$ oxidation state in their oxides is: $Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy$

The common stable oxidation states of $Eu$ and $Gd$ are respectively

The element californium belongs to the family of

$(i) La(OH)_3$ is the least basic among lanthanide hydroxides. $(ii) Zr^{4+}$ and $Hf^{4+}$ have almost identical ionic radii. $(iii) Ce^{4+}$ can act as an oxidizing agent. Which of the above statements are correct?

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:

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