Why are reactive in air even with full electrons in their d-orbitals?
Because of their small size.
Because of full electrons in their d-orbital.
Because of their low melting point.
Because of the reduction in the extent of intermetallic bonding.
The full electrons in their d-orbitals weaken the metallic bonding and make the metal more reactive as the number of electrons is loosely held and can be easily removed.
Hence option 4 is correct.