Final answer:
The correct fill-in-the-blank response is D) Decrease Increase. This reflects the normal age-related decrease in vestibular system hair cells, which leads to an increased difficulty with balance.
Step-by-step explanation:
With normal aging, the vestibular system shows a decrease in hair cells in the inner ear, leading to an increase in difficulty walking and standing on uneven surfaces. The correct answer to the fill in the blank question is option D) Decrease Increase. This occurs because the vestibular system, responsible for maintaining balance, uses hair cells located in the inner ear to detect head movements and gravitational forces. With a decrease in the number of these hair cells, the system becomes less effective, thereby increasing the difficulty in maintaining balance on uneven surfaces.