Which of these is a density-independent factor?
a. water
b. fire
c. food
d. space

The correct answer is Fire.

Any physical or biological factor whose impact is not dependent on the number of organisms living in that area ( population density) is referred to as the density-independent factor.

Fire is a density-independent factor because it will cause harm to all members of the population once the area starts burning.  

On the contrary, density dependent factors are those whose impact on the growth of population varies with the density of population.  

Water, food, and space are density-dependent factors. A specific number of organisms can occupy a particular space.  

Availability of food and water can impact the population of a particular area. This is because organisms may start dying in the scarcity of food and water. So they are also density-dependent factors.

Thus, fire is the correct answer to the density independent factor.