Even if it was difficult to predict, seeing (A) we see that it must be true in order for the conclusion to follow. If we use the negation test, we would have a situation where only one or two planes were scheduled for all nine flights. In that case, it wouldn’t be unlikely for mechanical problems to have caused all nine flights to be canceled, because mechanical problems for one or two planes are perfectly normal.

(A) reveals the author's equivocation between "flights" and "planes" in the argument. By making it explicit that the nine flights required more than two planes, (A) addresses a flaw we might not have spotted initially, but which is key to the argument.

The argument is limited to Swift Airlines, so the frequency of mechanical problems experienced by other airlines is irrelevant. We already know that Swift is unlikely to have mechanical issues with many planes on one day, so we don't need to go further by comparing to other airlines.

We don't know the other reasons that a flight may be cancelled, so there's no need to assume that the cancelled flights would have been unusually long. The length of the cancelled flights doesn't connect to anything else in the argument, so whether or not (C) is true doesn't make a difference.

(D) tells us that cancelling this many flights is in and of itself unusual, but that doesn't have to be true for the argument to make sense. Even if this had happened before, it wouldn't affect the conclusion, which is specifically about the likelihood of mechanical problems being to blame. Since it wouldn't affect the argument if (D) were false, it must not be necessary.

It doesn’t matter where the flights are based, because that doesn't affect the factors we're given by the stimulus. Whether or not the planes were based at the same airport, the flights were still cancelled, and it's still unlikely for the airline to have mechanical problems with many planes. The argument doesn't depend on an alternative explanation, so even if we think (E) might make the flights more vulnerable to other factors, e.g. inclement weather, that's still not necessary.