An increase in visible products of combustion is most likely due to:

The presence of excessive visible products of combustion is primarily associated with a limited amount of air available for combustion. When there isn't enough oxygen for complete combustion, the fire will not burn efficiently, leading to the production of more visible smoke, soot, and other unburned or partially burned byproducts. In a well-ventilated environment, adequate oxygen promotes efficient combustion, resulting in fewer visible products of combustion.

In contrast, options related to excessive ventilation and increased fuel availability typically facilitate more efficient burning processes, reducing the visual indicators of products of combustion. High humidity can potentially influence fire behavior, but it usually does not have as direct an impact on visible products of combustion as the availability of oxygen does. The critical factor in the formation of visible products of combustion is fundamentally tied to the sufficiency of air (oxygen) present during the burning process.