Answer: D. forest fire
Population density can be defined as the measurement of population of a species per unit area. It is the quantitative measurement noted in numbers. It is applied to all the living organisms and most commonly to humans. A limiting factor is the one which can limit the growth, abundance and distribution of a members of species in a habitat.
The forest fire is the limiting factor which is capable enough to decrease the population density of the species. As, fire the acquire a large surface area of a location or habitat and can kill a large number of individuals hence affecting the population density.