Which of the following best defines a greenhouse gas?

The definition of a greenhouse gas is best captured by the idea that it is a gas that contributes to the greenhouse effect. Greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, trap heat in the Earth's atmosphere, which leads to an increase in global temperatures. This process is essential for maintaining the Earth's climate, as it keeps the planet warm enough to support life. However, human activities, such as burning fossil fuels and deforestation, have increased the concentrations of these gases, enhancing the greenhouse effect and leading to climate change.

The other options do not accurately define greenhouse gases. While air pollution from motor vehicles may include some greenhouse gases, it does not encompass the broader definition or effects of greenhouse gases overall. Natural gas, found in underground reservoirs, primarily consists of methane, which is a greenhouse gas, but this option doesn't capture the full scope of what greenhouse gases are. Lastly, a chemical compound that does not interact with sunlight would not be considered a greenhouse gas, as the ability to absorb and emit infrared radiation is crucial to the greenhouse effect.