Final answer:
The reaction to the Earth's gravitational force when you stand on a scale is the force you exert on the scale, aligning with Newton's Third Law of Motion. While the scale displays in mass units, it actually measures the normal force, or your apparent weight.
Step-by-step explanation:
When you stand on a scale to measure your weight, the reaction to the force the Earth exerts on you is the force you exert on the scale (option c). This is an application of Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction. The Earth exerts a gravitational force on you, which is your weight, and you exert an equal force back on the Earth. However, what the scale measures is the normal force it exerts on you, which is equal to the gravitational force when not accelerating. Therefore, your apparent weight is actually the force you exert on the scale, which the scale measures and displays commonly in units of mass (kilograms). This measurement takes into account the local acceleration due to gravity, which is approximately 9.80 m/s2 on Earth’s surface.