Which of the following hydroxides is the most basic?

$4f^2$ electronic configuration is found in

The $Eu^{2+}$ ion is a strong reducing agent in spite of its ground state electronic configuration (outermost) : [Atomic number of $Eu=63$]

The number of $f$-electrons in the $+3$ oxidation state of gadolinium $(Z=64)$ is $x$ and in the $+2$ oxidation state of ytterbium $(Z=70)$ is $y$. The sum of $x$ and $y$ is:

$A$ member of the Lanthanoid series which is well known to exhibit $+4$ oxidation state is

Identify the correct decreasing order of atomic radii from the following.