The correct answer is option 2: thymosin. The thymus gland is a specialized organ in the immune system that is responsible for the production of T-lymphocytes, which are a type of white blood cell crucial for immune response. Thymosin is a hormone produced by the thymus gland that plays a vital role in the maturation and development of T-lymphocytes. It helps in the differentiation of T-cells into various subtypes that have specific functions in the immune system. Thymosin also aids in the formation and maintenance of healthy immune cells, enhancing the overall immune response. The other options, thyroxine, thvronine, and calcitonin, are hormones produced by different glands in the body and have different functions unrelated to the thymus gland.