Damage to thymus in a child may lead to:

Damage to the thymus in a child may lead to a loss of cell-mediated immunity. The thymus is the site of maturation of T-lymphocytes, which are a type of immune cell that plays a key role in cell-mediated immunity. So, the correct answer is option D. Notice that the other options are not correct. A reduction in haemoglobin content of blood is related to anaemia, which is not directly related to thymus damage. A reduction in stem cell production is related to bone marrow damage, which is not directly related to thymus damage. Loss of antibody-mediated immunity is related to B-lymphocyte dysfunction, which is not directly related to thymus damage.