Which of the following fuels produces the minimum amount of soot particles…

Soot (unburnt carbon particulate) forms when a fuel cannot mix completely with oxygen before combustion, leaving fuel-rich pockets where hydrocarbon chains break down (pyrolyse) into solid carbon instead of oxidising fully to CO2 and H2O. Two factors govern how much soot a fuel produces: its carbon-to-hydrogen (C:H) ratio — a higher C:H ratio means more carbon is available to form soot — and its physical state, since a gaseous fuel mixes with air far more uniformly than an atomised liquid spray and so burns more completely.

Application: Among the four options, Compressed Natural Gas (CNG) is almost pure methane (CH4), the simplest hydrocarbon and the lowest C:H ratio among common fuels. As a gas, it mixes uniformly with air before ignition, so combustion is far more complete and leaves minimal unburnt carbon — exactly why CNG is mandated for public transport in high-pollution cities in place of diesel.

• Petrol: a liquid hydrocarbon blend (roughly C4–C12) that must vaporise before burning; incomplete vaporisation in rich mixtures or cold starts still leaves some soot, though less than heavier fuels.

• Kerosene: made of longer hydrocarbon chains (roughly C10–C16) than petrol; the heavier chains burn less completely, producing visibly more soot — the classic sooty kerosene-lamp flame.

• Diesel: the heaviest common liquid fuel (roughly C10–C25); its compression-ignition combustion creates fuel-rich zones where unburnt hydrocarbons pyrolyse into carbonaceous soot particulates, giving diesel exhaust its high soot/PM output — the reason diesel vehicles need particulate filters.

Because CNG's low C:H ratio and gaseous mixing give the most complete combustion, it produces the minimum soot among the four fuels.