The correct answer is (D).
Remember that all flights will attempt to land in the most efficient manner possible which means they will travel through the fewest checkpoints necessary in order to land.
Because flights traveling at a speed of less than 405 knots cannot land, flight 15 will never land. Thus, (A) can be eliminated.
Flights 20 and 53 are much closer to checkpoint H than checkpoint O. Flight 96 is three checkpoints away from both H and O and, therefore, can land from either checkpoint.
Since flight 53 (534 knots) is traveling faster than flight 20 (444 knots), flight 53 will reach checkpoint H first. Thus, the landing order will be 53, 20, 96.
Note that flight 53 would land first even if flight 20 was flying at a faster speed because, according to the conditions, flight 53 needs to make an emergency landing.
Therefore, the correct answer is (D).