The Right Answer

This is a Critical Reasoning question, as indicated by the phrases "if true" and "undermines the argument" in the question. Remember our process in questions such as these:

Step 1: Identify the CONCLUSION of the argument.

Conclusion: The author thinks it is "unlikely" that the new fat-free foods will actually cause fat consumption to go down.

Step 2: Identify the REASON(S) the author has identified to support their conclusion.

Reason/Evidence: The author draws a comparison to sugar-free foods and highlights how it didn't work for these foods! Sugar consumption did NOT go down!

Step 3: Engage in the LOGICAL LEAP that undermines the conclusion. In most cases, this means attacking the evidence.

Possible Logical Leap #1: You might be tempted to undermine the author's conclusion by stating something like, "These fat-free foods are awesome, so people are going to buy them like crazy, and overall fat consumption will go down." The problem with this is that the author accounts for it. The author admits that "even if this is true...", so we cannot attack it from this angle.
A Better Logical Leap: Remember in many cases you're attacking not the conclusion of the paragraph but rather the evidence. In this case, let's attack the evidence the author provides about the sugar-free foods not reducing sugar consumption. One way to undermine the author's argument is to undermine their evidenceby stating, "Well the problem with that comparison you drew is the sugar-free foods SUCKED, so your comparison to the fat-free foods is baseless, especially because the paragraph clearly highlights that the fat-free foods are pretty awesome."
- Thus, the best answer choice is B. This is the answer choice that claims the sugar-free foods sucked.

The Wrong Answers

A: Okay, so manufacturers have several different kinds of fat substitutes. But this doesn't tell us whether they're any good or not. Perhaps they ALL suck, and nobody will buy any of them, which would of course support -- not undermine -- the author's argument.
C: This answer choice is mixing the ideas of "sugar-free" and "fat-free." The passage never states that a food item has BOTH of these properties. This answer choice is completely irrelevant.
D: This answer choice has the same problem as C -- it mixes the "sugar-free" and "fat-free" ideas together. That is something the passage doesn't do at all. In the passage, the foods are clearly separate.
E. If anything, this seems to support the author's argument rather than undermine it. If "not all foods" are available in the fat-free versions, then people will still have to buy foods with fat in them, thus supporting the author's argument that fat consumption is unlikely to be reduced.