Which of the following levels of organization is lower than the population?

The endosymbiotic theory which is also sometimes referred to as symbiogenesis states that all the organelles of the eukaryotic cells are prokaryotic cells that tends to live in a much larger host. This theory is important because it does explains how the eukaryotic cells are able to obtain the needed nourishment that is believed to be replenished by the prokaryotic cells.