The correct answer to the question is:
a. pia mater.
Here's why:
1. The meninges are three layers of protective membranes that surround the brain and spinal cord.
2. The three layers of the meninges are the dura mater, arachnoid membrane, and pia mater.
3. The pia mater is the innermost layer of the meninges and adheres closely to the surface of the brain and spinal cord, following its contours and providing support and protection.
4. In this question, the layer of the meninges that adheres to the outer surface of the brain is being asked. Since the pia mater is the layer that closely adheres to the brain's surface, it is the correct answer.
So, in this case, the correct answer is a. pia mater.
