Correct option is A
In brazing, flux is used to:
Prevent oxidation of base and filler metals,
Dissolve existing oxides,
Promote wetting and flow of the filler metal.
Among the options, the commonly used flux for brazing is borax.
Borax (sodium tetraborate, is a highly effective flux for high-temperature applications like brazing because:
It has excellent oxidation resistance at high temperatures.
It forms a glassy film that protects the metal surface from oxygen.
It helps filler metal to flow smoothly into the joint by lowering surface tension.
