Final answer:
While the scenario provided relates to physics and the calculation of forces on a ladder, the question about greenstick fractures is actually related to health. A greenstick fracture is more common in children due to their more flexible bones, so the person at highest risk of a greenstick fracture is age 8.
Step-by-step explanation:
To calculate the normal reaction and friction forces at the base of a ladder, we must consider the ladder and the person as a system and apply the principles of static equilibrium. Static equilibrium implies that all the forces and moments (torques) on the ladder are balanced. The forces acting on the system include the weight of the person, the weight of the ladder, the normal reaction at the base, and the friction force at the base.
The normal reaction force can be found by setting the sum of the moments around the base of the ladder to zero, while the friction force can be calculated by considering the sum of forces in the horizontal direction to be zero. Remember that the ladder is in static equilibrium, so the sum of horizontal and vertical forces, as well as the sum of moments (torques) around any point, should be zero.
While we can calculate these forces for a given setup, the original question seems to pertain to a biology or health-related question about greenstick fractures and not to the physics of a ladder. Greenstick fractures are more common in children, so the person at highest risk from the choices given would be age 8.