Final answer:
Hydrogen gas (H₂) is the common reducing agent listed, as it is known to donate electrons in chemical reactions, thereby being oxidized itself through the redox process.
Step-by-step explanation:
Among the options provided, hydrogen gas (H₂) is a common reducing agent. In chemical reactions, a reducing agent donates electrons to another substance and is thereby oxidized itself. The process involving a reducing agent is typically referred to as a reduction reaction, which is part of a redox (reduction-oxidation) reaction.
For example, during the reaction of hydrogen with copper(II) oxide (CuO), hydrogen can act as a reducing agent to convert CuO into metallic copper (Cu), while itself being oxidized to form water (H₂O). This illustrates the behavior of hydrogen as a reducing agent:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)
Other common reducing agents include substances like carbon (in coke or coal), carbon monoxide, and certain antioxidants such as ascorbic acid and vitamin E.