Answer:
The correct option is : d. astrocytes
Step-by-step explanation:
The blood brain barrier is a selective semipermeable border, which separates the extracellular fluid, present in the central nervous system and the circulating blood from the brain. This border is formed by the astrocytes, endothelial cells and the pericytes.
Astrocytes, collectively known as astroglia, are the star shaped glial cells present in the spinal cord and the brain. The astrocytic cells support the endothelial cells, in the blood brain barrier.