What is the purpose of an air dryer in a vehicle's air system?

Answer: B — To remove moisture from compressed air

The purpose of an air dryer in a vehicle's air system is to remove moisture from compressed air. In commercial vehicles, the air brake system relies on compressed air, and any moisture present can lead to several issues, including brake failure, freezing in cold temperatures, and corrosion in the air system components. The air dryer accomplishes this by utilizing a desiccant material that absorbs moisture from the air. By effectively eliminating moisture, the air dryer helps to ensure that the air brake system operates reliably and safely, contributing to overall vehicle performance and reducing maintenance needs. The other options address functions that are not related to the air dryer's role within the vehicle's air system. For instance, increasing air pressure in the tires pertains to tire inflation, regulating air temperature is more associated with heating or cooling systems, and filtering engine exhaust fumes relates to emission control systems rather than compressed air management.