Most accepted theory of ageing is
Immunity theory is the widely accepted theory for ageing. The immune system is programmed to decline its functions over time, and this fact is the main cause of aging. Thymic involution (shrinking of the thymus with age) is believed to be the main age-related change in the immune system, and this alteration begins after puberty.
So, the correct option is ‘Non functioning of thymus gland’.