The answer is (2) nucleus - nerve cell - brain - nervous system.
A nucleus is an organelle present in most of the body cells (red blood cells are an exception). The nucleus is present in nerve cells, as well. Nerve cells, besides nucleus, consist of many other organelles characteristic for animal cells. Billions of nerve cells work together and form a brain. The brain is an organ which is the part of the nervous system. The nervous system includes not only the brain but also a spinal cord and cranial and spinal nerves as well.