The correct options are
A Shock-absorbing fluid
B Formed primarily by choroid plexuses
C Lower in K+ and higher in Na+
Cerebrospinal fluid is a shock absorbing fluid which prevents the brain from bumping against the interior of the hard skull when the head is subjected to sudden, jarring movements.
It is formed by the choroid plexuses found in particular regions of the ventricle cavities of the brain.
The composition of CSF differs from that of plasma. For example, CSF is lower in K+ and higher in Na+, thus making the brain interstitial fluid an ideal environment for the movement of these ions down concentration gradients, a process essential for conduction of nerve impulses.
CSF is inside the cranium and not outside it.