Whether a variable is dependent or independent is decided on what you wish to obtain from a particular observation. In the case where you have no control over something or something is no effected by other changes, then that would be the independent variable. In the example, the question is referring to the effects on a moving object when the air resistance changes, so the speed would be the dependant variable. If you are changing the air resistance and then measuring the speed, the speed is dependent on the air resistance.
From the way the question is worded, at first glance it seems that air resistance is the Independent Variable .
"What is the effect of air resistance on the speed of a moving object?
But the amount of air resistance depends on the speed of the object, from the standpoint of physics.
More speed will create more air resistance, and the air resistance will (at least somewhat) inhibit the speed the object can attain.
If the object is a car - large and heavy - it seems to me that air resistance would have a very small effect on the car's speed.
If the object is something small and light, like a feather, air resistance would slow down its speed a lot.
In a vacuum, a feather and a cannon ball when dropped together, will fall at the same speed, correct? But with air resistance in the regular world, the feather will fall much more slowly than a cannon ball.
Statistics is not straight math - it's an application of math that has its own "peculiarities", I'll call them.
I didn't answer your question, did I? I think the answer will depend on what the object is, and on whatever other factors or constraints are involved in the experiment.