What does white or light grey smoke indicate during burning?

White or light grey smoke during burning is typically indicative of free burning. This type of smoke is often associated with the combustion of materials that are catching fire readily and burning quickly, such as certain types of wood or paper. The presence of white smoke suggests that there is sufficient oxygen for the fuel to burn effectively and that the combustion is taking place under relatively efficient conditions.

In contrast, the other options, while related to combustion processes, do not accurately describe the characteristics of white or light grey smoke. Incomplete combustion, for instance, usually produces darker smoke, indicating a lack of oxygen and the presence of unburned particles or gases. High efficiency combustion generally results in minimal smoke production, and high moisture content in the fuel can lead to heavy, white smoke due to the evaporation of water during the burning process, but it does not necessarily align with the notion of "free burning." Therefore, the correlation of white or light grey smoke with free burning provides a clearer understanding of what this smoke indicates in the context of combustion.