Final answer:
The highest level of complexity within an organism is the organism level, where all cells, tissues, organs, and organ systems work together to sustain life.
Step-by-step explanation:
The most complex unit within an organism is the organism level itself, which is at the top of the biological hierarchy of structural organization. An organism refers to a living being that consists of a cellular structure capable of performing all physiological functions necessary for life. In the case of multicellular organisms, such as humans, various levels of complexity exist below this, starting from atoms and building upward through molecules, organelles, cells, tissues, organs, to organ systems, which all work in an integrated manner to maintain the organism's life and health.