Which of the following does NOT express a degree of combustion?

The concept of combustion relates to the process of burning fuel in the presence of oxygen to produce energy, which can vary in efficiency and completeness. Degrees of combustion typically refer to how well the fuel is burned, and this includes categorizations like complete and insufficient combustion, based on whether all the fuel is utilized or there are unburned components remaining.

In this context, the term that does not express a degree of combustion is "constant." While complete, excess, and insufficient describe different conditions of how effectively fuel burns or how much oxygen is supplied relative to the fuel, "constant" does not have a relevant application to the efficiency or effectiveness of the combustion process. Instead, it simply implies a state of being unchanging, which does not pertain to combustive behavior. Thus, recognizing that "constant" does not relate to the combustion process helps distinguish it from terms that do indeed express varying levels of combustion.