The correct option is
A Decreased production of thyroxine
Myxoedema, also known as Gull's disease, occurs in adult due to deficiency of thyroid hormones after the mental and physical development of body is complete. It causes low metabolic rate, slow mental and physical process, coarse skin, loss of lustre from the hair and more hair fall.
So, the correct answer is 'Decreased production of thyroxine'.