Apaper airplane is thrown to the north from the top of a cliff with a speed of 2 km/h. the paper airplane lands 15 minutes (0.25 hours) later. how far from the cliff did the paper airplane land? a.0.25 km to the north b.0.5 km to the north c.0.7 km to the north d.1.5 km to the north

If you neglect air resistance, then you can solve for the horizontal distance using the formula:

dx=vixt

Where:
dx = horizontal distance
vix = initial horizontal velocity
t = time in flight

Now you can see that you have all the given you need. 
vix= 2km/h
t = 0.25 hours

We use hours because you need the units to be the same. So just plug that in your equation and you will have your

dx = vixt
     =(2km/h)(0.25h)
     =0.5km

This means that the answer to your question is B.