The atomic radius of elements of which of the following series would be nearly the same?

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:

Which of the following pairs of elements has the electronic configuration $(n - 1)d^{10} ns^2$?

In the $3d$ series of transition elements,the atomic radii from $Cr$ to $Cu$ are approximately the same because:

Which element has the least covalent radius?

What is the correct order of the theoretical magnetic moment for $Mn^{2+}$,$Cr^{2+}$,and $V^{2+}$?