Why not answer (3) is possible? Please briefly explain .

The thymus gland is located in the chest, behind the sternum. T-cells are produced in the bone marrow. These cells migrate to the thymus. The thymus gland is responsible for the differentiation and maturity of the T-cells. These mature T-cells are released in the bloodstream to fight against infections. The T-cells work in coordination with the B-cells to destroy the bacteria, viruses and other foreign antigens. If any damage is caused to the thymus during childhood may cause lack or less number of mature T-cells. This will cause the child to become more prone to infections. Thus, there will be lack or loss of cell-mediated immunity provided by the T-cells.?

So, the correct answer is option D?