While cooking, if the bottom of the vessel is getting blackened on the outside, it means that:
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.
When the fuel is not burning completely then sooty flame appears. This happens because of lack or incomplete supply of oxygen leads to incomplete combustion of fuel and makes the vessel black.
So, the correct answer is (b) - the fuel is not burning completely