Final answer:
The adult derivative of the wall of the telencephalon is the cerebrum, which is the largest region of the brain responsible for higher cognitive functions.
Step-by-step explanation:
The adult derivative of the wall of the telencephalon is the cerebrum. The telencephalon is one of the five secondary vesicles that develop from the primary vesicles in the embryonic brain. The telencephalon specifically develops into the cerebrum, which is the largest region of the brain and is responsible for higher cognitive functions.