Observe the following $f$-block elements: $Eu (Z=63)$; $Pu (Z=94)$; $Cf (Z=98)$; $Sm (Z=62)$; $Gd (Z=64)$; $Cm (Z=96)$. How many of the above have half-filled $f$-orbitals in their ground state?

If the ionic radius of $_{57}La^{3+}$ is $1.06 \ \mathring{A}$,then the ionic radius of $_{71}Lu^{3+}$ will be approximately close to which of the following values in $\mathring{A}$?

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

$Ce - 58$ is a member of

Given below are two statements:
Statement $I$: In the Lanthanoids,the formation of $Ce^{+4}$ is favoured by its noble gas configuration.
Statement $II$: $Ce^{+4}$ is a strong oxidant reverting to the common $+3$ state.
In the light of the above statements,choose the most appropriate answer from the options given below:

In the lanthanide series,the element well known to exhibit $+4$ oxidation state is