If an unknown substance cannot be broken down into a simpler substances, it is

1. a compound
2. an element
3. made of one kind of atom
4. both 2 and 3

4. both 2 and 3

To determine the correct answer, let's break down the options and understand the concepts involved.

1. A compound: A compound is a substance that is made up of two or more different elements chemically combined in fixed proportions. Compounds can be broken down further into simpler substances.

2. An element: An element is a pure substance that consists of only one type of atom. Elements cannot be broken down into simpler substances by chemical means.

3. Made of one kind of atom: This statement matches the definition of an element. Substances that are made up of only one type of atom are called elements.

Based on the definitions provided, the correct answer is option 4 - "both 2 and 3." If an unknown substance cannot be broken down into simpler substances and consists of only one type of atom, it is an element.