Final answer:
The value that can always be substituted for acceleration in Newton's second law, for an object in free fall on Earth, is 9.81 m/s², which is the acceleration due to gravity.
Step-by-step explanation:
In Newton's second law, the only value that can always be substituted for the acceleration of an object in free fall near the surface of the Earth is 9.81 m/s². This value represents the acceleration due to gravity, which is a constant for objects in free fall at or near the Earth's surface, assuming air resistance is negligible. Therefore, the correct answer to the question is c. 9.81 m/s².