Solid state welding :In these processes, the base materials to be joined are heated to a temperature below or just upto the solidus temperature and then continuous pressure is applied to form the welded

joint. No filler metal is used in solid-state welding processes.Forge welding (hot forge welding),cold welding, friction weldingand diffusion welding and are the types of solid state (solid phase welding).

Forge welding :In this welding process, the work-pieces (low carbon steel) to be welded are heated to the plastic condition (above 1000°C), and then placed together and forged while hot by applying force. Force may be applied by hammering, rolling, drawing or squeezing to achieve the forging action.

Hence, the correct option is (D).