Explanation :
When an object moves under the action of gravity. Then the object is said to be moving under free fall.
We know that the acceleration due to gravity is denoted by g and it is equal to
.
If the body is accelerating towards the gravity the value of g is positive while when it is accelerating against the gravity it is negative.
As the skydiver falls to Earth, she experiences positive acceleration due to gravity.
So, the correct option is (a) gravity.