**Carbon Dioxide**

Carbon dioxide is the compound identified from the given options. Let's understand why carbon dioxide is the correct answer.

**1. Oxygen:**
Oxygen is a chemical element, not a compound. It is represented by the symbol 'O' on the periodic table. Oxygen gas (O2) is a molecule composed of two oxygen atoms bonded together. However, in this case, the single element 'oxygen' is mentioned, which does not form a compound.

**2. Carbon Dioxide:**
Carbon dioxide (CO2) is a compound made up of two elements, carbon (C) and oxygen (O). It is formed when carbon combines with two oxygen atoms. Carbon dioxide is a colorless and odorless gas that is naturally present in the Earth's atmosphere. It is produced by various natural processes like respiration, volcanic eruptions, and combustion. Carbon dioxide is essential for photosynthesis, the process by which plants convert carbon dioxide and sunlight into oxygen and glucose.

**3. Hydrogen:**
Hydrogen is also a chemical element, not a compound. It is represented by the symbol 'H' on the periodic table. Hydrogen gas (H2) is a molecule composed of two hydrogen atoms bonded together. Similar to oxygen, the single element 'hydrogen' does not form a compound in this case.

**4. Air:**
Air is not a compound but a mixture of gases. It is composed of various gases, primarily nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and traces of other gases like argon, helium, and water vapor. Therefore, air cannot be considered as a compound.

In conclusion, carbon dioxide is the compound identified from the given options. It is formed by the combination of carbon and oxygen atoms and is represented by the chemical formula CO2.

Carbon dioxide contains only one type of molecule and the things that has one type of molecule are called compound.