Which forms interstitial compounds?

The correct order of the first ionization enthalpies is:

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 transition metal used in $X$-ray tubes is:

In which of the following is variable valency observed?

The metal ion (in gaseous state) with the lowest spin-only magnetic moment value is .... .