Which is the smallest gland in human body?

The pineal gland located in the epithalamus near the centre of the brain is the smallest gland in the human body with a length of about 5-8 mm. It is involved in modulation of the sleep pattern by secretion of melatonin and serotonin.

While liver is the largest gland of the human body.

Hence, option D. i.e., Pineal gland is the correct answer