There are a few things going on here. The plane is traveling horizontally, and it is also descending. Start by figuring out how far it travels horizontally. It moves in that direction at 9 feet per second for 5 seconds, so it moves horizontally 9×5 =45 feet. It descends 1 foot for every 3 traveled horizontally. If it goes 45 feet horizontally, it will descend more than 3 feet, so eliminate (A). Now figure out how many "3 feet" are in 45 feet-for each one of them, the plane will descend 1 foot. 45÷3=15, so the plane descends 15 feet, (C)
You may also have noticed that all the numbers in the question are odd. This makes it unlikely that the answer would be 10 or 20 , which are even. If you see things like that, use them as opportunities to eliminate.