A gland called 'Clock of ageing' that gradually reduces and degenerates in ageing is: Thyroid, Thymus, Parathyroid, Pituitary?

The thymus is the primary specialized lymphoid organ of the immune system. It gradually reduces with age. T cells or T lymphocytes mature within the thymus. Thymus reduces with age through a process called involution. This process has been observed in many vertebrates. This process starts during puberty; in middle age, the thymus changes from a primary lymphoid organ into fatty tissue. So, the correct answer is option B.