The correct answer is option 3: Ecosystem. The trophic level refers to the position of an organism in a food chain or food web. It represents the flow of energy and nutrients from one organism to another. The complexity of a trophic level depends on the number of organisms and their interactions within that level.

Option 1: Community refers to a group of populations of different species that live in the same area and interact with each other. While a community may consist of multiple trophic levels, it is not the most complex level itself.

Option 2: Population is a group of organisms of the same species living in the same area. It is a more specific level compared to community, but it does not involve interactions between different species.

Option 4: Species refers to a group of organisms that share similar characteristics and can interbreed. It is the most basic level and does not involve the complexity of multiple organisms interacting.

Therefore, option 3: Ecosystem, which includes multiple populations and their interactions, represents the most complex trophic level.