Final answer:
The most important lymphatic structure in an unborn child up until adolescence is the thymus, which is responsible for the maturation of T cells crucial for the immune system. Therefore correct option is C
Step-by-step explanation:
The lymphatic structure most important in an unborn child up until the age of adolescence is the thymus. The thymus is a bi-lobed organ located just above the heart, prominent in infants and becomes smaller in size through adulthood. It is here that T lymphocytes, or T cells, mature and are crucial for the developing immune system. The thymus gland produces hormones named thymosins which significantly contribute to the development of the immune response.
While tonsils and the spleen are also important lymphatic structures responsible for trapping pathogens and filtering blood respectively, it is the thymus that is particularly active during infancy and the pre-adolescent years, playing a pivotal role in the maturation of a key component of the adaptive immune system.