The answer is: b. population size
Population size refers to the actual number of total organisms that live in a certain area.
In order to predict the population size in a large area, Ecologists usually divide the area into smaller size (such as dividing the area by 10,000) and manually count the population size in that small divided area. After knowing the manual count, the ecologist would multiply it with 10,000 to make an estimation on the population size.