What defines perfect combustion?

Perfect combustion is characterized by the burning of all fuel in the presence of the exact amount of air required to completely oxidize the fuel. This means that the combustion reaction reaches its theoretical limits, leading to the most efficient reaction with no leftover fuel or unburned components.

When combustion occurs with the precise amount of air—known as stoichiometric air—every molecule of fuel reacts completely, producing only carbon dioxide and water vapor as byproducts. This provides a clear definition of perfect combustion as it emphasizes the complete consumption of fuel and generation of energy without any residues or incomplete reaction products.

In contrast, other scenarios such as using excess air can lead to inefficiencies and unwanted byproducts, while burning with insufficient air can create pollutants like carbon monoxide and soot. Thus, option B precisely captures the essence of perfect combustion as a theoretical condition, making it the correct choice.