Answer:
The correct answer is "thymus".
Step-by-step explanation:
The thymus is a lymphoid organ which main function is to produce T cells used for protection in the immune system. While most organs grow until maturity, the thymus stops growing at puberty and starts to shrink throughout adulthood. The reason of the shrinking is that the thymus as a reduced role in adulthood because most of the T cells are produced during childhood. As the thymus shrinks, its tissue is replaced by adipose tissue.