The correct option is
A Thymus
A. Adrenal gland is present on upper side of kidneys and has two parts, adrenal cortex which secrete glucocorticoids, mineralocorticoids and sexocorticoids and adrenal medulla which secrete adrenaline and nor-adrenaline hormone.
B. Pineal gland is a small, rounded and reddish brown gland present at the dorsal side of forebrain. This gland secretes melatonin and serotonin hormones.
C. Thymus gland is associated with the development of immune system and present on the dorsal side of heart. It secretes thymosin hormone which plays an important role in the differentiation of T-lymphocytes. This gland shows maximum development in children during puberty and after that, it starts diminishing gradually and by the age of 70, it becomes a tiny structure.
D. Pituitary gland is the master gland that regulates the activity of other endocrine glands and secretes many hormones like growth hormone, follicle stimulating hormone, thyroid stimulating hormone etc.
Hence, thymus becomes inactive after puberty.
So, the correct answer is 'Thymus'.