Answer- B (Third nerve)The third nerve (Oculomotor) does not form the floor of the third ventricle although it lies in close relation to the floor.The anterior wall of the third ventricle is formed by Lamina terminalis, Anterior commissure, Anterior columns of the fornixThe posterior wall is formed by the Pineal body, Posterior commissure, Cerebral aqueduct.The roof is formed by Body of fornix and the ependyma lining the undersurface of the tela choroidea of the third ventricle.The floor of the third ventricle is formed by Optic chiasma, Tuber cinereum, Infundibulum, the Mammillary bodies, Posterior perforated substance, and Tegmentum of the midbrain. The lateral wall is formed by the Medial surface of the thalamus, Hypothalamus, and the Hypothalamic sulcus.