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

Assertion: In general,transition metals have high melting points.
Reason: More number of electrons from $(n-1)d$ and $ns$ are involved in interatomic metallic bonding.
The correct option among the following is

Although $Zr$ belongs to the $4d$ transition series and $Hf$ belongs to the $5d$ transition series,it is quite difficult to separate them. Why?

Assertion : Transition metals show variable valency.
Reason : Transition metals have a large energy difference between the $ns^2$ and $(n-1)d$ electrons.

State the reason for the stability of the $Gd^{3+}$ ion.

$Zn$ and $Cd$ metals do not show variable valency because