Final answer:
The decrease in T cell immunity at puberty is due to atrophy of the thymus, also known as thymic involution, which reduces the gland's output of mature T cells, crucial for cell-mediated immunity. So, the correct option is B) atrophy of the thymus.
Step-by-step explanation:
The decrease in cell-mediated (T cell) immunity at puberty is primarily due to the atrophy of the thymus. This process is known as thymic involution, which begins at birth and continues until middle age, reducing the thymus gland's size and output of mature T cells. The reduced production of T cells due to thymic involution weakens cell-mediated immunity, as T cells are essential for this arm of the immune response. Mature T cells are required for the targeting and destruction of cells infected with intracellular pathogens, highlighting the critical role of thymic health in maintaining robust immunity.