This is a fairly straight forward problem. It is asking us to find a principle that, when added, will justify the conclusion.
Premise: Consumers attempting to lose-weight will equate "sugar- free" with "low in calories."
Premise: Manufacturers are aware that consumers are misled.
Conclusion: Labeling of high calorie foods as "sugar-free" based on all sugar by artificial sweeteners should be prohibited by law.
The conclusion is strong by stating that it SHOULD be prohibited so we should look for an answer choice that states that we should, indeed, do that. Unfortunately, in this particular problem, all answer choices mention what should be done.
A: This issue with A is that it states that the labels are literally incorrect when in fact they are literally correct. They do not contain sugar which does in fact make them "sugar-free." Therefore, this answer choice can be eliminated fairly quickly.
B: Same as A.
C: This starts out fine by suggesting that the labeling is literally correct but goes awry by mentioning an expert. The stimulus doesn't mention that an expert will impact the decision making process for buying foods. Eliminate.
D: This starts of good by saying that the labeling is literally correct. Additionally, this answer choice mentions that manufacturers intentionally misrepresent the products which result in people potentially being harmed.
E: Nowhere in the stimulus does it suggest that only two possible interpretations are available.