Ok, its not weight because that is our own natural thing, and has nothing to do with force, so you can eliminate that.
Its not inertia because it is 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. So we can go ahead and eliminate that choice.
Gravity - the force that attracts a body toward the center of the earth, or toward any other physical body having mass. For most purposes Newton's laws of gravity apply, with minor modifications to take the general theory of relativity into account.
So your answer would be Gravity.
Hope I helped. :)
