Niobium $(Nb)$ and ruthenium $(Ru)$ have " $x$ " and " $y$ " number of electrons in their respective $4d$ orbitals. The value of $x+y$ is $...........$

Observe the following statements:
$a$) The basic strength of $Lu(OH)_3$,$Gd(OH)_3$ and $Ce(OH)_3$ follows the order: $Ce(OH)_3 > Gd(OH)_3 > Lu(OH)_3$
$b$) $O^{2-}$,$N^{3-}$,$F^{-}$ and $Na^{+}$ are isoelectronic species
$c$) The sizes of $Zr$ and $Hf$ are approximately the same
The correct statements are:

Platinum,Palladium and Iridium are called noble metals because

Choose the correct statement regarding transition elements.

Which of the following pairs does not consist of isoelectronic species? (Atomic numbers: $Sm = 62, Er = 68, Yb = 70, Lu = 71, Eu = 63, Tb = 65, Tm = 69$)

Which of the following pairs has similar ionic radii?