Innate capacity of a population to increase in size.
a. mortality
b. intrinsic rate of growth (r)
c. natality
d. realized niche

b. Intrinsic rate of growth (r)

Explanation:

The per capita rate of increase in the population size makes its intrinsic growth rate. The maximum intrinsic growth rate is exhibited by a population when all the required resources are available and there are no negative ecological interactions such as competition. Each population has its specific innate capacity to increase its size that is represented by "r ".  

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