What is the outermost membrane surrounding the brain called?

pia mater

arachnoid mater

dura mater

alma mater

dura mater

Explanation:

The brain has three membranes between the bone, and the brain, the meninges: dura mater, arachnoid mater and pia mater.

Dura mater is the most external one, closest to the skull.

Arachnoid mater is the one in the middle. Looks like a spider-web, and that's why the name.

Pia mater is in touch with the brain and spinal cord, this is the thinner one.