Answer:
The correct answer is option "population".
Step-by-step explanation:
In ecology, a population is defined as a group of organisms living in a particular area or region. An observation of a herd of elephants will be a study at the population level, since it is comprised of a limited number of individuals all belonging to the same species. A study of a community would involve observing different species and their interactions, while a study of an ecosystem would involve studying living and non living components over an extensive geographical region.