Final answer:
Gravity exerts force on the skateboarder due to her position relative to the ground, and this force is due to the gravitational potential energy. As the skateboarder moves, this potential energy converts into kinetic energy and other forms.
Step-by-step explanation:
The potential for gravity to exert force on a skateboarder is due to the skateboarder's position relative to the ground. The correct answer to the question is (b) Gravity exerts force on the girl due to her position relative to the ground. This force is a result of gravitational potential energy, which is energy stored because of an object's position in the gravitational field of Earth. The force of gravity on an object is calculated by the formula F = mg, where F is the gravitational force, m is the mass of the object, and g is the acceleration due to gravity (approximately 9.8 m/s² on Earth's surface).
When a skateboarder is at a height, such as at the top of a ramp, they have gravitational potential energy due to their elevated position within the Earth-object system. Once the skateboarder begins to move down the ramp, this potential energy is converted to kinetic energy, and other forms of energy as well, like heat from friction. The gravitational potential energy depends on the vertical distance from the reference level, which is often taken as Earth's surface.