Final answer:
Argon is the commonly used shielding gas that can be used alone or in combination with other gases.
Step-by-step explanation:
The commonly used shielding gas that can be used alone or in combination with other gases is argon (option A).
Argon is one of the noble gases and it is known for its stable outer subshell, which makes it unreactive and resistant to ionization. Because of its low reactivity, argon is often used as a shielding gas to protect the weld pool during welding processes. It helps to prevent oxidation and contamination of the weld.
For example, in Tungsten Inert Gas (TIG) welding, argon is commonly used as a shielding gas to provide a stable and inert atmosphere around the electrode and the weld pool.