The endocrine structure that secretes the hormone promoting T-cell formation in the bone marrow is the?

The thymus is the correct choice because it is an essential gland in the immune system responsible for the maturation of T-cells, which are crucial for the body's adaptive immune response. T-cells originate from bone marrow and migrate to the thymus, where they undergo a selection process to ensure they function properly in identifying and combating pathogens. The thymus secretes hormones, particularly thymosin and thymopoietin, that promote the maturation of T-cells.

In contrast, the thyroid gland primarily regulates metabolism and calcium homeostasis through hormones like thyroxine and calcitonin, and does not play a direct role in T-cell formation. The pituitary gland is often referred to as the "master gland" because it regulates various hormonal functions in the body but is not involved in T-cell maturation. The adrenal glands produce hormones like cortisol and adrenaline, which are involved in stress response but do not directly influence T-cell development in the bone marrow. Thus, the thymus stands out as the specific endocrine structure responsible for the secretion of hormones that promote T-cell formation.