Final answer:
The longer taxi cab drivers have spent in their profession, the larger the size of the hippocampus in their brains, as seen in research using Structural MRI.
Step-by-step explanation:
The correct answer is d) Structural MRI, hippocampus.
Research using Structural MRI has shown that the longer taxi cab drivers have spent in their profession, the larger the size of the hippocampus in their brains. MRI scans use a strong magnetic field to generate detailed images of the brain's structure. In this study, the size of the hippocampus was measured using Structural MRI, and it was found to be larger in taxi cab drivers who had more experience.