CO2 is the chemical symbol for:

Carbon dioxide is represented by the chemical formula CO2, indicating that each molecule consists of one carbon atom bonded to two oxygen atoms. It is a naturally occurring gas in the Earth's atmosphere and is produced by various processes, including respiration, combustion, and the decomposition of organic matter. It plays a crucial role in the carbon cycle and is used by plants during photosynthesis to produce oxygen.

The other choices do not match the molecular composition of CO2. Carbon monoxide has the formula CO, indicating one carbon and one oxygen atom. Hydrocarbon is a broad class of compounds composed of only carbon and hydrogen but does not refer to a specific substance represented by CO2. Carbonic acid, which has the formula H2CO3, is formed when carbon dioxide is dissolved in water but is distinct from carbon dioxide itself. Understanding these distinctions helps clarify the significance and characteristics of carbon dioxide in various scientific and environmental contexts.