Final answer:
A person is less stable standing than sitting because in a standing position, the center of gravity is higher and the base of support is narrower, making it easier for small disturbances to displace the center of gravity outside the base of support.
Step-by-step explanation:
The statement which explains why a person in the standing position is less stable than a person in a sitting position is: A. The center of gravity is higher and the base of support is narrower. This is because when standing, one's center of gravity is above the hip joints and lies between narrowly-separated feet, resembling a pencil on its eraser. Such a position is in stable equilibrium for sideways displacements, but becomes unstable for small displacements forward or backward, as the feet are not long and a small disturbance may move the center of gravity outside the base of support. To increase stability, humans often unconsciously widen their stance or bend their knees, which both lowers the center of gravity and widens the base of support, as demonstrated by athletes preparing for a play.