While cooking, if the bottom of the vessel is getting blackened on the outside, it means that:

The correct option is A: the fuel is not burning completely

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that fuel is not burning completely. On complete burning of a hydrocarbon fuel in oxygen, will primarily yield carbon dioxide and water. Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. Partially oxidized fuel produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.