Hmm, tricky one. I think it's all in the language.
Slots do "dump" "positive pressure airflow" from under the wing, by transfering that air on to the top of the wing.
To answer that "They dump positive pressure airflow from under the wing", and leave it there, suggests that positive pressure airflow under the wing causes the stall, and therefore "dumping" it will somehow delay the stall. This misses the point, which is that it is the lack of "positive pressure airflow" on the top of the wing which causes the stall. The reason slots work is because of the change to the airflow on the top of the wing.
Therefore, although b) is technically correct, I would say that it is missing a vital piece of information, and d) is more correct. But I think it's a pretty nasty set of answers to give to choose between.
FFF
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