Concept:Fuels burn with a flame only if they vaporize easily to form combustible gases that mix with air.
Explanation:Solid fuels like coal often glow or smolder without a visible flame because they do not vaporize.
Liquid fuels with high calorific value may or may not produce a flame depending on their volatility.
Fuels containing carbon and hydrogen can produce a flame if they vaporize, but many such fuels (e.g., solid hydrocarbons) do not.
The key property is volatility or the presence of volatile substances.
Volatile fuels vaporize at low temperatures, forming flammable vapours that ignite and produce a steady flame.
Examples: petrol, natural gas, propane – these burn with a clear flame.
Non-volatile fuels like charcoal or coke burn without a distinct flame.
Hence, the correct choice is the one mentioning volatility.
Answer:B. Those which are volatile or contain volatile substances.