Thymus: The Key to T-Cell Development and Immunity

The correct answer is Thymus.

The thymus is an endocrine gland located in the upper chest. It is responsible for the maturation of T lymphocytes, which are a type of white blood cell that plays a vital role in the body's immune system. The thymus produces hormones that promote the development of T cells and help them to distinguish between the body's own cells and foreign invaders.