Which of the following gases is NOT typically emitted from a vehicle's exhaust?

Freon is a refrigerant commonly used in air conditioning systems and is not emitted from a vehicle's exhaust. When discussing vehicle emissions, the focus is on byproducts generated from the combustion of fuel in the engine.

Carbon dioxide (CO2), nitrogen oxides (NOx), and hydrocarbons are all standard emissions resulting from the combustion process. Carbon dioxide is a product of complete combustion, nitrogen oxides are formed during high-temperature combustion, and hydrocarbons are unburnt fuel that escapes the combustion process. In contrast, Freon, being associated with refrigeration and cooling systems, is not produced during the operation of a vehicle's engine, hence it is not found in exhaust emissions. This distinction helps in understanding the different chemical compounds involved in vehicular operation versus those used in other automotive systems.