Which of the following best describes force in physics?

In physics, force is fundamentally defined as any interaction that, when unopposed, will change the motion of an object. This means that a force can cause an object to start moving, stop moving, or change its direction and/or speed. When a force is applied to an object, it can change its state of motion, which directly correlates with the concept of acceleration as laid out by Newton's second law of motion (F=ma).

The other options do not accurately describe force. The amount of substance refers to mass, which is a different concept related to how much matter is present in an object but not directly to the interactions that change motion. Energy pertains to the capacity to do work, but it does not define force itself. Lastly, gravitational pull describes the weight of an object, which is a specific type of force but does not encapsulate the broader definition of what force is in physics. Hence, the most fitting definition of force in this context is indeed an interaction that causes an object to change velocity.