Organize the parts of this body system in the correct order according to the levels of organization in a multicellular organism, simplest to most complex..

If you are referring to levels of biological organization, then lets start by talking about he different levels from simplest to most complex.

Cells: This is the simplest and building blocks of life. 
Tissues: It is a group of cells that work together and serve a specific function.
Organs: It is a group of tissues that work together and serve a specific function.
Organ System: Is a group of organs that work together to do a specific biological process.
Organism: This is a living organism that is composed of organ systems that function together.

There are other levels but your question refers to a multi-cellular organism, so we will stop there. Here is an example, which might also be the question you are referring to:

Neurons - nerve tissue - brain - central nervous system - human being 
 Cell               Tissue          Organ        Organ System                  Organism

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