Correct : b

CSF (Cerebrospinal Fluid) is a clear, colourless fluid that cushions and protects the brain and spinal cord. It is present in the subarachnoid space — the space between the arachnoid mater and the pia mater, which are two of the three meningeal layers surrounding the brain and spinal cord.
The other options are wrong for clear anatomical reasons. The subdural space lies between the dura mater and the arachnoid mater — it is a potential space that normally contains only a thin film of serous fluid, not CSF. The epidural space lies between the dura mater and the vertebral bone — it contains fat and blood vessels and is clinically used for epidural anaesthesia but does not contain CSF. The pericardial space surrounds the heart entirely and has no connection to the nervous system or CSF at all.
CSF is produced by the choroid plexus inside the ventricles of the brain, flows through the ventricular system, enters the subarachnoid space through openings called the foramina of Luschka and Magendie, circulates around the brain and spinal cord, and is finally reabsorbed into the venous blood through the arachnoid granulations.
Correct answer: B — Subarachnoid space