Answer:
Population
Step-by-step explanation:
Population is a group of individuals that live in the same geographical area which have the ability to interbreed because they belong to the same species. So organisms of the same species when live in a particular area are considered in population but members of the same species that are geographically isolated and can not interbreed do not make a population.
Many population comes together to form community and many communities together forms an ecosystem. An ecosystem is a functional unit which is made up of many communities with interacts with its physical environment. So the correct answer is population.