The correct option is
B Thymus
The thymus is a specialized primary lymphoid organ of the immune system. The T-cells are produced from the bone marrow. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is the largest and most active during the neonatal and pre-adolescent periods.
So, the correct answer is 'Thymus'.