The element that usually does $NOT$ show variable oxidation states is

Which element from the following in $+2$ oxidation state exhibits the highest magnetic moment?

Which of the following statements is incorrect?

Which one of the following ions has the same number of unpaired electrons as those present in $V^{3+}$ ion?

The "spin-only" magnetic moment [ in units of Bohr magneton,$(\mu_B)$ ] of $Ni^{2+}$ in aqueous solution would be (At. No. $Ni = 28$).

The increase in the atomic radii of the third $(5d)$ series of transition elements is very small,which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are: