The correct option is D pia mater
The brain is surrounded and supported by three protective tissue layers collectively called the cranial meninges. They are (from outside to inside) dura mater, arachnoid and pia mater, respectively. The dura mater, which is the outermost layer of the cranial meninges, is found to be very thick and tough. The very thin middle layer is called the arachnoid mater because of the presence of various spider-like processes called arachnoid trabiculae that is found to extend to the third layer from this layer. Pia mater is the thin innermost layer of delicate cells. Many curves and folds are found on the surface of this layer. Pia mater is the layer which is in contact with the brain tissue.