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In the context of flammable gases and ignition, normal concentrations of oxygen in the air play a critical role. For a substance to ignite, it must be present in an environment that has the right conditions, and one of the most vital conditions is sufficient oxygen to support combustion. Flammable gases require an atmosphere with a certain concentration of oxygen to sustain the chemical reactions that lead to ignition.

The presence of atmospheric oxygen allows for the necessary oxidation reaction to occur when a flammable gas encounters an ignition source, such as a spark or flame. If the concentration of oxygen is too low, even highly flammable gases may not ignite. Therefore, the normal concentrations of oxygen found in the air significantly contribute to the potential for flammable gases to ignite, making this the correct choice for the question.

In contrast, while factors such as flash points, weight, and temperature are important in the broader context of combustion and flammability, they do not directly impact the critical aspect of ignition initiated by the presence of oxygen.