To answer this question, we need to understand the concept of pollution caused by different fuels when they are burnt.
Option A) Petrol - Petrol is a fossil fuel that is commonly used in vehicles. When burnt, petrol releases carbon dioxide (CO2), a greenhouse gas that contributes to climate change. It also releases nitrogen oxides (NOx) and particulate matter, which contribute to air pollution. Therefore, petrol causes pollution when burnt.
Option B) Diesel - Diesel is another fossil fuel commonly used in vehicles and industrial machinery. When burnt, diesel also releases carbon dioxide (CO2), nitrogen oxides (NOx), and particulate matter. Similar to petrol, diesel causes pollution when burnt.
Option C) Coal - Coal is a fossil fuel primarily used for electricity generation and industrial processes. When burnt, coal releases a significant amount of carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter, and mercury. These emissions contribute to air pollution and have detrimental effects on human health and the environment. Therefore, coal causes significant pollution when burnt.
Option D) Natural gas - Natural gas is a fossil fuel primarily composed of methane (CH4). When burnt, natural gas releases carbon dioxide (CO2) and water vapor. Compared to other fossil fuels, natural gas has the lowest carbon content, which means it produces fewer carbon dioxide emissions when burnt. It also produces lower amounts of other pollutants, such as sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter, compared to coal, petrol, and diesel. Therefore, natural gas causes the least pollution when burnt.
The correct answer is Option D) Natural gas. This option is correct because natural gas produces the least pollution when burnt, compared to the other options.