The correct statements are:
a) No flux is used in gas welding of mild steel.
b) Borax is the commonly used flux coating on welding electrodes.

Gas welding of mild steel:
Gas welding is a process that uses a flame produced by a mixture of fuel gas and oxygen to melt the base metal and filler rod in order to join two or more pieces of metal together. In the case of mild steel, no flux is used during the gas welding process. Flux is a substance that is used to prevent oxidation and contamination of the molten metal during welding. However, in gas welding of mild steel, the high temperature flame generated by the fuel gas and oxygen mixture provides sufficient protection against oxidation and contamination, eliminating the need for flux.

Flux coating on welding electrodes:
Welding electrodes are often coated with a flux material to enhance the welding process. Flux coating serves multiple purposes, such as removing impurities, improving the weld bead appearance, and protecting the molten weld pool from oxidation and atmospheric contamination. Borax, a compound containing boron, is commonly used as a flux coating on welding electrodes. Borax helps in removing oxides and other impurities from the surface of the base metal, allowing for a cleaner and stronger weld joint. It also helps in preventing the formation of porosity and other defects in the weld.

Explanation of incorrect options:
c) Laser beam welding and vacuum chamber:
Laser beam welding is a high-precision welding technique that uses a focused laser beam to melt and join the base metal. However, laser beam welding does not require a vacuum chamber. In fact, laser beam welding is often performed in ambient air or with the use of shielding gases to protect the molten metal from oxidation and contamination.

d) AC for GTAW process:
GTAW (Gas Tungsten Arc Welding), also known as TIG (Tungsten Inert Gas) welding, is primarily performed using DC (Direct Current) power supply. The use of DC allows for better control of the welding process and improved stability of the arc. While AC (Alternating Current) can be used for certain applications in GTAW, such as welding aluminum, it is not the commonly used power supply for this process.

In summary, the correct statements are that no flux is used in gas welding of mild steel, and borax is the commonly used flux coating on welding electrodes.