Organ system is the answer according to me if it is one word question
The atmosphere is a mixture because it contain many types of gases , liquids and solids.
In order to study the formation of cancer cells, you should study the life cycle of normal cells as cancer cells are abnormal cells.
Actually, Normal cells grow, divides and died after an interval of time. And when this process is break down, cancer cells begin to form. They are usually old cells which don't die after their life cycle.
Hope this helps!
