Infundibulum develops from ?

Ans. is 'c' i.e., Floor of third ventricle(Ref: Textbook of Neuroanatomy p. 112)The infundibulum of the diencephalon is a downward evagination of the floor of the third ventricle.As the infundibulum grows down, it meets an ectodermal diveiculum of the oral cavity, known as Rathke's pouch.