Final answer:
Argon is the most commonly used shielding gas in plasma arc welding due to its inert properties and abundance in the atmosphere.
Step-by-step explanation:
The most commonly used shielding gas in plasma arc welding is argon (option 2). Argon is one of the noble gases, specifically from Group 18, which are monatomic and unreactive at room temperature and pressure. Because of argon's low reactivity, it serves as an excellent inert atmosphere for processes that require protection from the surrounding air, such as welding. It is preferred for this purpose over gases like hydrogen, which are more reactive and can pose safety concerns. Furthermore, argon is the third most abundant gas in the Earth's atmosphere, making it conveniently available for industrial use.