Which characteristic best identifies a decomposition reaction?
1. a single compound as a reactant
2. a single compound as a product
3. an element and a compound as reactants
4. an element and a compound as products

Option (1) is the correct answer.

Explanation:

A decomposition reaction is defined as the reaction in which a compound splits or breaks into two or more number of atoms.

For example,  

This means that a single compound as a reactant is splitting in a decomposition reaction with two or more number of products.

Thus, we can conclude that a single compound as a reactant, characteristic best identifies a decomposition reaction.

where's the diagram?

the answer is a and have a great day