The correct answer is option 2: stem cells. Stem cells are the basic units from which human spare parts can be created. Stem cells are unique because they have the ability to develop into different types of cells in the body, including nerve cells, heart cells, kidney cells, and many others.
Option 1: Nerve cells are specialized cells found in the nervous system and are responsible for transmitting signals between different parts of the body. While nerve cells are important for the functioning of the body, they are not the basic units for creating spare parts.
Option 3: Heart cells are specialized cells found in the heart and are responsible for pumping blood. Like nerve cells, they are not the basic units for creating spare parts.
Option 4: Kidney cells are specialized cells found in the kidneys and are responsible for filtering waste products from the blood. Again, they are not the basic units for creating spare parts.
Therefore, the correct answer is option 2: stem cells, as they have the potential to differentiate into various types of cells and can be used to create spare parts for the human body.