Final answer:
A compound is a chemical substance that can be broken down into elements, making it the correct answer to the question.
Step-by-step explanation:
The question asks which substance can be broken down into another substance. Among the given options, a compound is a chemical substance that can be broken down into another substance (elements). A compound is a pure substance that can be broken down into chemically simpler components because it is made up of more than one element. For example, water (H₂O) is a compound that can be separated into hydrogen (H) and oxygen (O) through a chemical reaction such as electrolysis.