Data about immigration and emigration of individuals of a species' population are helpful to determine how big a population is in each space of its distribution since accounting for how many individuals left that group of individuals, and how many come to it, is useful to address if the population of a particular place is in an ecological equilibrium or not. This leads us to consider that the correct answer is the third one: C. population of a species. We can see an example of this in the following picture about migratory patterns of some bird species: