Catalytic converters work to reduce all of the following except:

Catalytic converters are designed to convert harmful emissions produced by internal combustion engines into less harmful substances before they are released into the atmosphere. They primarily target three main pollutants: carbon monoxide, hydrocarbons, and nitrogen oxides.

The correct answer is grounded in the function of catalytic converters, which facilitate chemical reactions that reduce the toxicity of emissions. Carbon monoxide, a poisonous gas that can be harmful to human health, is converted to carbon dioxide. Hydrocarbons, which contribute to smog and can be harmful to the respiratory system, are oxidized to form carbon dioxide and water. Likewise, nitrogen oxides—pollutants that can cause acid rain and contribute to ground-level ozone—are converted into nitrogen and oxygen, which are harmless.

In contrast, carbon dioxide is not a target for reduction because it is a naturally occurring component of the Earth's atmosphere and is also produced in the normal process of combustion. While catalytic converters can transform harmful emissions into carbon dioxide, they do not reduce the amount of carbon dioxide emitted; instead, they convert carbon monoxide and hydrocarbons into carbon dioxide as part of their function. Thus, while catalytic converters help reduce several harmful pollutants, they do not reduce carbon dioxide, making it the correct answer in this context.