The correct answer is (a) Convection heating
To explain: Convection heating is not a heating source in metallization. In resistance heating, aluminium wire is fed onto a block of metal that is heated by holding it in a carbon crucible or boat across which a high current is arched. A pool of melted metal forms in the boat cavity and evaporates into a cloud above the source. In induction heating, heating is done by passing a high current through heating coils, causing an induced magnetic field inside the coils. In electron beam heating, the source is heated by electron beam.