The liquefied metal which expands on solidification is

Although Zirconium belongs to the $4d$ transition series and Hafnium to the $5d$ transition series,they show similar physical and chemical properties because . . . . . . .

The catalytic activity of the transition metals and their compounds is ascribed to their

The electrons which take part in order to exhibit variable oxidation states by transition metals are

The correct option with the order of melting points of the pairs $(Mn, Fe)$, $(Tc, Ru)$ and $(Re, Os)$ is $:$

Identify the incorrect statement among the following.