Transition elements show high melting points. Why?

Why do the transition elements exhibit variable oxidation states?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Generally, $3d$ transition metals have high melting points.
Reason $R$: Involvement of $3d$-electrons in addition to $4s$-electrons in the interatomic metallic bonding.
In light of the above statements, choose the most appropriate answer from the options given below:

The electronic configuration of $Pt$ (atomic number $78$) is $...$

If $A$ is $[Rn]\,6d^2\,7s^2$,then identify the block,period,and group.

The maximum oxidation state of $Cr$ is: