Final answer:
The most complex level to the simplest level in a multicellular organism is organ system, organ, tissue, cell.
Step-by-step explanation:
The correct answer is B. organ system, organ, tissue, cell.
In a multicellular organism, cells are organized into tissues, tissues are organized into organs, and organs are organized into organ systems. Finally, organ systems work together to form the whole organism.
Therefore, the most complex level is the organ system, followed by organs, tissues, and then cells, which is the simplest level. For example, in the human body, the respiratory system is an organ system, the lungs are an organ, alveoli are a tissue, and the cells in the alveoli are the simplest level.