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

the fuel is not burning completely.

The correct answer is option 2. the fuel is not burning completely.

When the bottom of a cooking vessel is getting blackened on the outside, it indicates that the fuel is not burning completely. Incomplete combustion of the fuel can lead to the production of soot (carbon particles), which can accumulate on the bottom of the vessel and cause it to become blackened. This is a common occurrence, especially with solid fuels like wood or coal, and it suggests that the combustion process needs improvement to achieve more complete burning of the fuel.