Option B is correct. According to the textbook context, when the bottoms of cooking vessels get blackened, it indicates that the air holes are blocked, which limits the supply of air. This limited air supply results in incomplete combustion of the fuel, producing a sooty flame. The context clearly states that 'incomplete combustion gives soot which is carbon' and that 'limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame.' The blackening is caused by soot (carbon) depositing on the vessel. Options A, C, and D are incorrect as the context does not relate blackening to food being uncooked, wet fuel, or complete combustion.