What is measured during an emissions test?

During an emissions test, the primary focus is on the concentration of pollutants in the exhaust gases emitted by a vehicle. This testing procedure is crucial for assessing how well a vehicle complies with environmental regulations concerning air quality.

By measuring the levels of various pollutants—such as carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HC), and particulate matter—technicians can determine whether the vehicle's emission control systems, like catalytic converters and exhaust gas recirculation systems, are functioning effectively. This ensures that the vehicle meets the required standards set by governmental regulations aimed at reducing air pollution.

The other options pertain to different vehicle attributes that are not relevant to emissions testing. Fuel quantity is not an indication of emissions performance, the speed of the vehicle does not correlate directly to emission levels, and the vehicle's age, while it may influence overall emissions due to wear on components, is not directly measured during an emissions test. The focus remains solely on the pollutants produced and their concentrations in the exhaust, making the identified choice the most appropriate answer.