What is defined as a mixture of solid particles and liquid droplets suspended in the air?

Particulate matter is specifically defined as a mixture of solid particles and liquid droplets suspended in the air. These particles can vary in size and composition, including dust, dirt, soot, and liquid droplets containing various chemicals. Particulate matter plays a significant role in air quality and can have various health effects when inhaled, as it can penetrate deep into the lungs and even enter the bloodstream.

Other choices, such as carbon monoxide, sulfur dioxide, and nitrogen dioxide, are all gases rather than a mixture of solids and liquids. These gases are significant pollutants in their own right, but they do not fit the definition of a mixture of solid particles and liquid droplets. Thus, the definition clearly aligns with particulate matter, making it the correct answer.