The correct answer is option 3: stem cells. Stem cells are a type of cell that have the ability to develop into any type of cell in the body. They are undifferentiated cells, meaning they have not yet specialized into a specific type of cell. Stem cells can be found in various tissues and organs in the body, such as bone marrow and umbilical cord blood.

Option 1: Endodermal cells are a type of cell that make up the inner layer of tissue in many organs, such as the lining of the digestive tract. These cells are already specific to their function and cannot develop into any other type of cell.

Option 2: Ectodermal cells are a type of cell that make up the outer layer of tissue in many organs and structures, such as the skin and nervous system. Like endodermal cells, they are already specialized and cannot develop into any other type of cell.

Option 4: Muscle cells are a specialized type of cell that make up muscle tissue. They are responsible for contraction and movement. Muscle cells cannot develop into other types of cells.

In conclusion, only stem cells have the unique ability to differentiate into any type of cell in the body.