T-cells develop in the __________.

T-cells, or T-lymphocytes, are a type of white blood cell that plays a crucial role in the adaptive immune response. They originate from precursor cells in the bone marrow but undergo their development primarily in the thymus. During their time in the thymus, T-cells mature and differentiate, gaining the ability to recognize specific antigens and develop important functions like killing infected host cells and regulating other immune cells.

The thymus is essential for the selection process that ensures T-cells can appropriately respond to pathogens while avoiding reactions against the body's own tissues. This maturation process involves a series of interactions with thymic stromal cells that teach T-cells the difference between self and non-self, which is critical for preventing autoimmune reactions.

In contrast, while the bone marrow is where T-cell precursors originate, it is not the site of their development into fully functioning T-cells. The liver and spleen play different roles in the immune system; for example, the spleen filters blood and helps activate immune responses but does not support T-cell maturation. Thus, the thymus is the correct answer as it is the specific organ where T-cells undergo significant maturation and functional specialization.