Negating (A) would give us: "Training programs cannot eliminate pilot errors." But the author's argument doesn't require training programs to eliminate pilot errors. The focus is on reducing pilot errors, not getting rid of them completely.

Irrelevant. Even if we negated this answer choice to say that commercial pilots do not undergo routine additional training, this would (if anything) strengthen the author's case for rethinking the design of the initial training programs for pilots and making sure they have more flying experience from the outset. Since negating (B) doesn't destroy the argument, this isn't the answer we're looking for.

This is a tricky answer choice. Notice that nothing in the argument requires the author to believe that a training program focusing on increasing actual flying time alone will decrease the number of airline crashes. The author isn't saying the other factors mentioned — classroom hours and cockpit communication — aren't important, just that they can't make up for a lack of flying time; an effective training program might have to include these factors, as well as other focuses, in addition to increasing flying hours. In other words, while the author assumes that addressing a lack of flying time is a necessary condition in a training program to reduce airline crashes, her argument doesn't require increasing flying hours alone to be sufficient to decrease airline crashes, as (C) is saying.

Correct. If this assumption isn't true — i.e., if lack of actual flying time is not an important contributor to pilot error in plane crashes —then the author's only reason for rejecting the airlines' current training program, that it doesn't compensate for a lack of flying time, becomes irrelevant. Since negating (D) would destroy the argument, we can see that (D) is a necessary assumption.

Incorrect. The author never assumes that communication skills are unimportant. She says that the focus on classroom hours and communication skills cannot compensate for a lack of flying hours, but doesn't say that the first two things are not also important or necessary parts of a training program.