The correct option is D Both (a) and (b)
The thymus gland, located behind your sternum and between your lungs, is only active until puberty.
It serves a vital role in the training and development of T-lymphocytes or T cells, an extremely important type of white blood cell.
Thymosin is the hormone of the thymus, and it plays a major role in the proliferation and differentiation of T- lymphocytes, which provide cell-mediated immunity. It also promote production of antibodies to provide humoral immunity and hastens sexual maturity.
Therefore the removal or absence of thymus in early life results in a lack of lymphocytes as well as antibodies.