Air resistance, also known as drag, is the force that opposes the motion of an object as it moves through the air. Therefore, among your provided options, the correct description is:
The force that opposes motion (though it sounds like you meant "the force that opposes falls" – air resistance does oppose the downward motion of falling objects, making them fall more slowly).
It's important to note that air resistance acts in the opposite direction to the motion of the object, making it harder for the object to accelerate downward (in the case of falling) or forward (in the case of moving through the air).
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