What is the chemical symbol for oxygen?

The chemical symbol for oxygen is represented as "O." However, the choice "O2" specifically refers to molecular oxygen, which consists of two oxygen atoms bonded together and is the form of oxygen most commonly found in the Earth's atmosphere. This diatomic molecule (O2) is critical for life as it is used by living organisms during respiration.

The other choices, while related to oxygen or its compounds, do not represent the elemental form of oxygen itself. "O3" denotes ozone, which is a molecule made up of three oxygen atoms and has different properties and functions compared to molecular oxygen. "CO2" stands for carbon dioxide, a compound made of one carbon atom and two oxygen atoms, important in respiration and photosynthesis but not a representation of elemental oxygen. "H2O," or water, contains hydrogen and oxygen atoms but does not directly represent oxygen in its elemental form.

Therefore, selecting "O2" is appropriate when considering the molecular representation of the oxygen we breathe.