The impulse is given by the magnitude of the force times the time interval:

We can see that the force F of the 4 skateboard is always the same (33 N), this means that the skateboard with greatest impulse is the one with the largest time interval, so skateboard C).
<em>Anything</em> that's dropped through air is somewhat affected by air resistance. But, out of that list, the leaf and the balloon are the items that will be affected by air resistance enough so that you can plainly see it.
If you spend some time thinking about it, you can kind of understand why airplane wings and boat propellers are shaped more like leafs and balloons than like bricks and rocks.