In lanthanides,with increase in atomic number,atomic radius decreases,except for the element $\underline{ X }$. What is $\underline{ X }$?

$Ce$ $(Z=58)$ is a member of

Which statement is incorrect for the electronic configuration $[Xe]\, 4f^{7}\, 5d^{1}\, 6s^{2}$?

Lanthanoids are

The element californium belongs to the family of

Among $Ce(4f^{1} 5d^{1} 6s^{2})$,$Nd(4f^{3} 6s^{2})$,$Eu(4f^{7} 6s^{2})$ and $Dy(4f^{10} 6s^{2})$,the elements having highest and lowest $3^{rd}$ ionisation energies,respectively are