The correct answer is a) ecosystem.
An ecosystem is the highest level of organization among the options provided. It includes all living organisms (plants, animals, microorganisms) in a particular area, along with their physical environment (such as water, soil, and climate). Within an ecosystem, you can find communities, which are groups of different populations that interact with one another. A population, on the other hand, refers to a group of individuals of the same species living in a given area. A division is not a commonly used term in ecology for levels of organization. Therefore, the ecosystem is a comprehensive term that encompasses communities and populations, making it the correct choice.