What term describes the degree to which exhaust gas blocks light?

The degree to which exhaust gas blocks light is referred to as opacity. Opacity measures how much light is obstructed by a substance; in this context, it specifically relates to the visibility and density of exhaust gases. The greater the opacity, the less light can pass through, making the exhaust appear darker or more visible in the air. This measurement is important in environmental monitoring and regulation because it can indicate the amount of pollutants in the air, affecting air quality and public health. Other terms provided in the options do not pertain to the ability of a substance to block light in the same manner. For example, viscosity relates to a fluid's resistance to flow, clarity refers to the clearness of a substance, and reflectivity pertains to how much light is reflected off a surface rather than how much is blocked.