kaplan official explanation:
Identify the Question Type
Since it asks to undermine an explanation, this is a Weaken question.
Untangle the Stimulus
The stimulus contains a causal argument: the desire to prove that fast food is not junk food caused the fast-food chains to introduce the low-fat and low-carb menu items. The author assumes that the fast-food restaurants changed their menus for this reason and no other.
Predict the Answer
Predict that the correct answer will present a different possible cause for the introduction of the new items.
(D) provides an alternative explanation: if the chains believe they can make more money on these new items, that may be reason enough to introduce them, regardless of any health claims.
(A) is incorrect; which of the two types of food is better for consumers is irrelevant to why the chains are offering the items in the first place.
(B) offers no alternative explanation, because the new items were not intended to replace other items completely, but merely to show that not all fast food is junk food.
(C) is irrelevant; people who will never eat at these chains regardless of what they offer are not the cause of any change in menu offerings.
(E) provides some evidence that chains have made other menu changes because of the perception of unhealthiness, so it certainly doesn't weaken the argument that this perception wasn't the reason for the new menu items.
TAKEAWAY: To weaken a causal argument, look for evidence of another cause.