Final answer:
The most likely density-independent cause of mortality among the options given is C. Fire, as it affects population mortality regardless of population density.
Step-by-step explanation:
The cause of mortality most likely to be density-independent among the options A. Competition for food, B. Disease, and C. Fire is C. Fire. Density-independent factors typically include physical or chemical elements, such as weather, natural disasters, and pollution, that affect a population's mortality regardless of its density.
Thus, a forest fire will cause mortality whether the population density of an affected species is high or low; its impact does not depend on the number of individuals in the area.