The answer is (d) circulatory system.
In the human body, the circulatory system is a fractal.
The various organs of the body need blood for their functioning. Our heart makes it possible by pumping the blood through the large arteries that enter the organs. Inside the organs, there are several tissues, and the artery branches into smaller arteioles to provide blood to all these tissues. Tissues are further made up of cells, the arteioles branch into much smaller blood capillaries to provide blood to the cells. The circulatory system is a fractal because there's an infinite length of blood vessels within a finite boundary of the body or the organs.