Which of the following glands is large sized at birth but reduces in size with ageing?

The correct option is C Thymus
Pineal:

Pineal gland releases melatonin which plays a role in the sleep wake cycle. Its size does not get reduced on aging.


Pituitary:

Pituitary gland is located at the base of the brain and is known as a master endocrine gland that controls hormone release from other endocrine glands. Its size does not get reduced on aging.


Thymus:

Thymus gland located in the chest, in between the lungs behind the sternum are the primary lymphoid organs. The T cells undergo maturation in the thymus. As age advances, the size of the thymus is reduced. This causes decreased immunity in the elderly and aged persons.


Thyroid:

Thyroid gland is located in the neck and produces hormones including $T_4$,$T_3$ and calcitonin. Its size also doesn’t get reduced on aging.


Final answer c. Thymus