The correct option is 2, 3, and 4 because the Air Quality Index (AQI) primarily reflects the concentration of pollutants that have well-established health effects on humans and the environment. Carbon monoxide (CO), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂) are critical indicators of air pollution due to their toxicity, contribution to respiratory problems, and association with urban sources of pollution. In contrast, carbon dioxide (CO₂) and methane (CH₄) are greenhouse gases primarily linked to climate change rather than immediate air quality, thus they are not typically included in AQI calculations. This distinction underlines the purpose of the AQI to inform the public about health risks associated with short-term exposure to specific air pollutants.