The correct answer is c) astrocytes.
Astrocytes are star-shaped glial cells in the brain and spinal cord that play several vital roles, one of which is the maintenance of the blood-brain barrier (BBB). This barrier is crucial for protecting the brain from potentially harmful substances in the bloodstream while allowing essential nutrients to pass through. Astrocytes help form this barrier by wrapping their end-feet around blood vessels and regulating the flow of ions and nutrients between the blood and neurons. In contrast, microglial cells act as the immune defense in the central nervous system, oligodendrocytes are responsible for the myelination of neurons, and ependymal cells line the cavities of the brain and spinal cord. Understanding the role of astrocytes underscores their importance in both brain health and disease.