The correct answer is option 3, embryo.

Stem cells are undifferentiated cells that have the ability to develop into different types of cells in the body. These cells have the potential to become specialized cells such as muscle cells, nerve cells, or blood cells. They also have the ability to divide and replicate themselves to generate new cells.

Embryonic stem cells are derived from embryos that are a few days old. These embryos are typically surplus embryos from in vitro fertilization procedures. Because embryonic stem cells are pluripotent, meaning they have the ability to differentiate into any type of cell in the body, they are considered a valuable resource for scientific research and potential medical applications.

In contrast, the other options listed in the question are not sources of stem cells. Roots (option 1) are part of plants and do not produce stem cells. Stem (option 2) refers to the stem of a plant and is also not a source of stem cells in the context of human biology. Flower (option 4) is a reproductive structure of a plant and does not produce stem cells.