The correct answer is a- Intact blood brain barrier to plasma protein.

The CSF/Serum Albumin index is a measure of the integrity of the blood-brain barrier (BBB), which is a highly selective barrier that separates the circulating blood from the brain and spinal cord tissues. The BBB is composed of tightly packed endothelial cells that line the cerebral blood vessels and restrict the passage of large molecules, such as plasma proteins, from the bloodstream into the brain.

When the BBB is intact, the CSF/Serum Albumin index is low, indicating that there is minimal leakage of plasma protein into the cerebrospinal fluid (CSF). A CSF/Serum Albumin index of less than 9 is considered normal and indicates an intact BBB.

On the other hand, if the BBB is compromised, plasma proteins can leak into the CSF, leading to an increase in the CSF/Serum Albumin index. A CSF/Serum Albumin index greater than 9 indicates a breakdown of the BBB and can be seen in various neurological conditions such as multiple sclerosis, meningitis, and brain tumors.

In summary, a CSF/Serum Albumin index of less than 9 indicates an intact BBB and suggests that there is minimal leakage of plasma protein into the CSF.

Reference: 
1. Frohman EM, Racke MK, Raine CS. Multiple sclerosis--the plaque and its pathogenesis. N Engl J Med. 2006 Mar 9;354(10):942-55. doi: 10.1056/NEJMra052130. PMID: 16525042.