What signifies a substance being in free fall?

A substance is considered to be in free fall when it is only influenced by gravitational force. In this state, the only significant force acting upon the object is gravity, resulting in an acceleration equal to the acceleration due to gravity (approximately 9.81 m/s² near the surface of the Earth). This condition implies that there is no resistance acting against the motion of the object, such as air resistance, which would otherwise affect its acceleration.

In scenarios where an object is moving in a circular path, it is experiencing centripetal acceleration, which is not a characteristic of free fall. Likewise, an object with zero acceleration is not in free fall; it would either be at rest or moving at a constant velocity, indicating the absence of any net force acting on it, including gravity. Finally, when significant air resistance is present, this opposing force would alter the object's motion, meaning it is not truly in free fall, as it is not solely influenced by gravity. Thus, the correct choice indicates that free fall occurs under the influence of gravity alone, with no other forces acting on the object.