You are correct that D is the correct answer as that is what the passage is trying to support. That salt makes foods more prone to having bacteria after microwave oven use and that lack of salt would allow the microwave to kill these bacteria.
On the other hand, answer choice E stretches this too far. This is why it would be incorrect. Despite salt being a reason for food poisoning despite microwave use, we have no idea whether it is the primary cause or not.
Be on the look out for answer choices that exaggerate or make statements that are too strong.
I hope that helps. Please let me know if you have any other questions.