Answer:
A Animals,
Step-by-step explanation:
Complexity deals with the level of organization of life an organism belongs to. There are different levels of organization of life and as you move up, the complexity increases.
Cell → Tissues → Organs → Systems → Organism
As we go through this levels from the left to the right, the complexity of the organisms found at each levels increases.
Most animals exists at the organism level and they are made of several systems which enables them to function properly.