Final answer:
The odd one out is D) pia mater, as it is a protective meningeal layer covering the brain, not a part of the brain itself like the cerebrum, cerebellum, and diencephalon.
Step-by-step explanation:
The odd one out from the options A) cerebrum, B) cerebellum, C) diencephalon, and D) pia mater is D) pia mater. While the cerebrum, cerebellum, and diencephalon are all parts of the brain itself, responsible for processes like memory, coordination, and sensory relay, respectively, the pia mater is quite different. The pia mater is a meningeal layer, one of the three that make up the meninges, which are the protective coverings of the central nervous system. The cerebral cortex is enclosed by these layers of meninges, comprising the pia mater, arachnoid mater, and dura mater. Specifically, the pia mater is the innermost layer that directly covers the brain and spinal cord, offering protection and housing blood vessels that nourish the brain.