The third ventricle is a narrow (unpaired) cavity that lies between
Pons varolii and medulla oblongata
Brain and spinal cord
Cerebellum and hypothalamus
Two cerebral hemispheres
Third ventricle or Diocoel:
The third ventricle is a narrow (unpaired) cavity that lies between the cerebral hemispheres.
Specifically, this ventricle lies in the diencephalon where it is enclosed laterally by the hypothalamus and thalamus.
The third ventricle is connected to the fourth ventricle through the cerebral aqueduct (aqueduct of Sylvius).
Hence, the correct answer is option d.