The correct answer is B.
As there are four flights, the penultimate flight to land will be the flight that lands third. 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.
The position of all four flights after each has moved will be:
20: G → H
39: F → I
41: T → O
74: R → N
Now, flight 39 can land first. Therefore, the position of the four flights will be:
20: H → M
39: I → runway (lands first)
41: O → I
74: N → O
Now, flight 41 can land second. Therefore, the position of the three remaining flights will be:
20: M → R
41: I → runway (lands second)
74: O → I
Now, flight 74 can land third. Therefore, the position of the remaining two flights will be:
20: R → N
74: I → runway (lands third)
Thus, flight 74 lands third.