This question checks whether we know which fusion welding process keeps the surrounding base metal from being disturbed by heat, that is, which process gives the smallest Heat-Affected Zone (HAZ). Let's look at each option.
Submerged Arc Welding (SAW): This process runs a high current arc under a layer of flux. The high current means a large amount of heat goes into the joint, and this heat spreads into the base metal, giving a fairly wide HAZ.
Tungsten Inert Gas Welding (TIG): TIG gives a cleaner, more controlled arc than SAW, so its HAZ is narrower than SAW's. It still uses an electric arc that covers a noticeable area, so the HAZ is not as small as it can get.
Electron Beam Welding (EBW): EBW fires a stream of electrons that is focused down to a very fine spot using magnetic lenses, inside a vacuum chamber. This gives an extremely high power density in a tiny area, so the weld happens fast with very little extra heat leaking sideways into the base metal.
Oxy-Acetylene Welding (OAW): This uses a gas flame, which is spread out and much less intense than an arc or a beam. Because the flame heats a bigger area for a longer time, this process gives the widest HAZ of the four.
Since EBW packs the most energy into the smallest spot and finishes the weld fastest, it leaves the least time and area for heat to spread into the base metal. So EBW gives the smallest Heat-Affected Zone.
Let's summarize:
Narrower HAZ comes from higher energy density and faster welding speed.
Ranked from narrowest to widest HAZ: EBW, then TIG, then SAW, then OAW.
So the fusion welding technique with the least Heat-Affected Zone is Electron Beam Welding (EBW), option (C).
