Explanation:

- Argon is a noble gas. It is found in the far right column of the periodic table.

- Noble gases are generally unreactive. They exist as single atoms in their natural state.

- Monoatomic substances are made of single atoms. This means they do not bond with other atoms in their natural state.

- Polyatomic substances consist of multiple atoms, which can be of the same or different elements.

- Diatomic molecules contain two atoms, either of the same or different elements. Common examples are oxygen (O2) and nitrogen (N2).

- Triatomic molecules have three atoms, such as ozone (O3).

- Option 1: Monoatomic is the correct classification for argon as it exists naturally as individual atoms.