Which among the following lanthanoids has the smallest atomic size?

Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

The $f$ orbitals are half-filled and completely filled,respectively,in which of the following lanthanide ions?
(Given: Atomic numbers: $Eu=63, Sm=62, Tm=69, Tb=65, Yb=70, Dy=66$)

State the $E^o$ values of lanthanoids and provide their reactions with the following: $(i)$ $H_2$,$(ii)$ Carbon,$(iii)$ Dilute acid,$(iv)$ Halogens,$(v)$ $O_2$,and $(vi)$ $H_2O$.

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:

Name the member of the lanthanoid series which is well known to exhibit $+4$ oxidation state.