Which of the following is monoatomic?

The correct answer is option 4. Helium.

Helium is the only monoatomic gas among the options provided. Monoatomic gases consist of individual atoms that are not bonded to each other. Helium, as a noble gas, exists in nature as individual atoms and does not form molecules. It is chemically inert and exists as a monoatomic gas under normal conditions.

Nitrogen (option 1), oxygen (option 2), and chlorine (option 3) are diatomic gases, meaning their molecules consist of two atoms bonded together.