Newton's Second Law of Acceleration says it is gravity.
Gravity
Acceleration of a falling object is caused by the force of gravity. Gravity is a force that pulls objects towards the center of the Earth, causing them to accelerate downward at a rate of 9.8 m/s^2.
It reduces the acceleration of the falling object due to friction.
The acceleration of a falling object is called gravity. A free-falling object has an acceleration of 9.8 m/s/s when going downward on Earth.
Assuming that there are no other forces on the object, the force that causes the acceleration of a falling object is the gravitational force (attractive force that exists between two masses). In problems, this assumption is usually used.However, in Force = mass*acceleration it is important to remember it is net acceleration and net force. Thus, for an object falling in real life the acceleration is caused by the gravitational force and a drag force which results from the object moving through the air. You have to take into account all the forces on the object.
Acceleration is caused by a change in an object's velocity, either in speed or direction. This can occur due to the application of a force to the object, such as gravity or a push or pull from another object.
Gravity
If you are ignoring wind and other variables then the upward acceleration is negative velocity caused by gravity. So in most cases (-9.8 m)/(s^2)
Acceleration due to gravity is the rate at which an object accelerates towards the Earth when falling freely under gravity. It has a constant value of approximately 9.81 m/s^2 near the surface of the Earth.
0 because while the mass remains at 16 Kg, as the object is falling its weight (caused by the pull of gravity on its mass) becomes 0 as its acceleration equals that of the acceleration due to gravity. (This is why things seem weightless when in orbit round the Earth - they are actually falling).
acceleration
the acceleration of gravity is 9.8 m/s