Final answer:
A person must either lean forward or slide their feet under the chair to stand up in order to keep their body in equilibrium by moving their center of gravity within their base of support.
Step-by-step explanation:
The reason a person must either lean forward or slide their feet under the chair to stand up is D. To keep the body in equilibrium while rising. Standing up from a seated position requires shifting the center of gravity (cg) forward and over the feet to maintain balance. When seated, the body's cg is positioned over the chair. To rise, one must move their cg forward, either by leaning or by moving their feet back, to bring it within their base of support. This allows for equilibrium as the person rises, as the body must be balanced over the feet to avoid falling. The muscles work to stabilize the body throughout this movement.
It's not to increase the force required to stand up, use friction with the ground, or reduce the energy required to stand up. Instead, it's about ensuring the body's stability and balance by adjusting the cg within the base of support provided by the feet. This principle also relates to other activities that require balance, such as riding a bicycle where stability is achieved when the force exerted by the ground is aligned with the cg