Conclusion :- market for breath freshening products will decline as the effectiveness of the tongue scraper becomes more widely known.
Weaken :- market for breath freshening products will not decline as the effectiveness of the tongue scraper becomes more widely known.
We are looking for a flaw in argument :-
The flaw in the argument hinges on a implied causal relationship between people knowing certain information, and changing their behaviour. Specifically, the argument is that if people know that tongue scraping is a good way to eliminate bad breath, they will stop purchasing other products (mints, gums, and sprays) for eliminating bad breath.
Answer D directly addresses the flaw:
A recent survey shows that 94% of those who regularly purchase breath freshening products are aware of the effectiveness of the tongue scraper.
Answer D undermines the notion of a causal relationship between knowledge and behaviour change. It says the opposite: that most people who buy other products DO KNOW that tongue scraping is a good way to eliminate bad breath (but they buy the other products anyway).
Answer E is more of a follow up and counterpoint to answer D than it is evidence of a flaw in the passage. Maybe those people like to buy products that are green, for example. That "fact" does not point out a flaw in the argument, it just suggests a reason why people buy the other products.