During the fall of a body freely under the action of gravity alone in a vacuum, the only force acting on the body is gravity. Since there are no other forces involved, the work done by gravity is the only change in energy experienced by the body.

Option 1: Potential energy - As the body falls, its height decreases, resulting in a decrease in potential energy. Therefore, potential energy is not constant during the fall.

Option 2: Kinetic energy - As the body falls, its speed increases. Kinetic energy is directly proportional to the square of the velocity. Since the velocity is increasing, the kinetic energy is also increasing. Therefore, kinetic energy is not constant during the fall.

Option 3: Total linear momentum - Linear momentum is the product of mass and velocity. As the body falls, the velocity increases, resulting in an increase in the linear momentum. Therefore, total linear momentum is not constant during the fall.

Option 4: Total mechanical energy - The total mechanical energy of a system is the sum of the potential energy and kinetic energy. Since both potential energy and kinetic energy change during the fall, the total mechanical energy also changes. Therefore, total mechanical energy is not constant during the fall.

Alert - correct answer should be None of the above