Hey there!
When we look at major organs and organ systems in the body, like the nervous system and brain, it's hard to realize the components it consists of.
We start with a cell. A tiny, microscopic cell. It will serve as a building block for everything thereafter.
Cells- lots of cells- create tissues. Tissue is just an organized group of the same and similar cell. Tissues then come together and work together to create an..
Organ- Organs are groups of tissues, and examples of them are the lungs and heart (muscular organ). They help you and work together to make sure you're alive and doing well.
Organ System- Organ systems are groups of organs, like the circulatory or nervous system. They control main functions of your body and all work together.
Organism- An organism is an animal, plant, or living thing- like people. All those organs and organ systems composed of tissues and cells are all there and keep us alive. They create an organism
All in all, our order is:
Cell -> Tissue -> Organ -> Organ System- Organism
That means our answer is A.
Hope this helps!