Which clue can be used to identify a chemical reaction as a decomposition reaction? The reaction involves oxygen.
The reaction involves ions.
The reaction has a single reactant.
The reaction has a single product.

The reaction has a single reactant

Explanation:

Decomposition reaction is a reaction in which an insoluble single compound is heated at a high temperature and decomposes into several products.

Typically, we may see decomposition of calcium carbonate producing calcium oxide and carbon dioxide, this is represented by the following reaction:

Another example would be decomposition of ammonium nitrite. Ammonium nitrite would decompose into nitrogen and water:

Notice that the two examples above share something in common: the two decomposition reactions have a single reactant.