Answer : 2. "Air Quality Index "
Explanation :

The AQI, or Air Quality Index, is a numerical scale used to communicate the quality of air in a specific location. It provides information about the level of various air pollutants, such as particulate matter (PM), ground-level ozone, sulfur dioxide, carbon monoxide, and nitrogen dioxide. The AQI is typically used by government agencies and environmental organizations to inform the public about the potential health effects of the current air quality. The scale is divided into categories, ranging from "Good" to "Hazardous," each corresponding to a different level of health concern. Monitoring and reporting AQI helps people make informed decisions about outdoor activities and take precautions when air quality is poor.