Final answer:
The complete elementary reaction among the given options is the direct formation of water from hydrogen and oxygen gases, represented by 2 H₂(g) + O₂(g) → 2 H₂O(g), which is a synthesis or composition reaction. So option (2) is correct.
Step-by-step explanation:
The student has provided a series of reactions and is asking to identify the complete elementary reaction amongst them. A complete elementary reaction can be defined as a single-step process in which the reactants are converted to products without any intermediate states or reactions. The options presented involve the reaction between hydrogen (H₂) and oxygen (O₂) gases to form water (H₂O).
The correct option in this case is the one that represents the direct formation of water from hydrogen and oxygen, without implying any intermediate steps or reverse processes. Here is the correct reaction written out:
2 H₂(g) + O₂(g) → 2 H₂O(g)
This reaction states that two molecules of hydrogen gas react with one molecule of oxygen gas to form two molecules of gaseous water. This is a synthesis reaction, also known as a composition reaction, where two or more substances combine to form a single product. Since the reactants directly form the product without any other intermediates or reversibility implied, this is considered a complete elementary reaction.