Final answer:
C) Cerebral cortex. The cerebral cortex is the brain structure that helps show that dolphins have the ability to complete complicated tasks.
Step-by-step explanation:
The brain structure that helps show that dolphins have the ability to complete complicated tasks is the cerebral cortex. The cerebral cortex is the outermost part of the cerebrum and is highly convoluted and folded in mammals, including dolphins. This increased folding provides a greater surface area for the cortex, which is correlated with complex behavior and intelligence.