As the skydiver falls to earth, she experiences positive acceleration due to
your

gravity.
inertia.
friction.
air resistance.

It is gravity. As a sky diver falls (ignoring air resistance for now) he increases in acceleration due to gravity, or 9.81m/s^2 (Earth's gravitational acceleration). Inertia is, by definition, a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force (credit to Google). However, if the sky diver has some kind of webbing on his suit, a drag force opposite to gravitational weight cancels it, and it moves at a constant drag velocity.

Sorry I got carried away... physicist...

It is gravity. Hope this helps!

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