Final answer:
Brain shrinkage most affects memory and multitasking.
Step-by-step explanation:
Brain shrinkage can affect various cognitive functions, but the function most commonly affected is memory. Memory includes processes such as remembering bus routes and recalling information. Multitasking, which requires the ability to juggle multiple tasks and remember different aspects of each task, can also be significantly impacted by brain shrinkage. Therefore, the correct answer is D) multitasking.