The argument is fine but the question and choices are very non GMAT like.

IMO C

The survey is about dentists and the conclusion is about consumers.
In case the consumers do not care about the dental health, the argument falls apart.

I agree with Charu that it is not GMAT like question. None of the answer logically stands out. Please share the source.

@ Charu : Even C is wrong because the statement says " IF" you care about dental health, which means that it is applicable only to people who care about dental health.

The argument structure is as follows:

Conclusion: if you care about great taste and great dental health, trust the experts and pick up Galactico gum
Premise 1:Galactico gum is the only gum proven to help bad breath and gingivitis, statements that have been evaluated and approved by the federal health authority
Premise 2: 90% of the dentists we surveyed state that they prefer Galactico gum to any of its competitors
The question asks us to find dubious reasoning in the passage. Let's look at the options:

The method used to survey the dentists is not given. - This is an example of dubious reasoning. The argument uses the survey data as a premise to back up the conclusion; however, it doesn't give the method used to select dentists. If all the dentists selected were the ones who had some ties with the Galactico company, then the survey results would be meaningless.

Great taste and great dental health need not go together. - Irrelevant. In the conclusion, there is an 'if' statement; so whether these go together or not doesn't affect the argument at all.

Few consumers care about great dental health - 'If' statement in the conclusion precludes those customers who don't care about great dental health. Thus, even if there are only few consumers who care about great dental health, the argument remain intact.

The dentists might also recommend a number of other gums. - The survey results clearly state that dentists prefer Galactico over other gums. Thus, there is no lack of clarity here.

Galactico gum may not be to everyone's liking. - Nowhere it is claimed that is will be to everyone's liking.

Thus, the correct option is A.

Cheers,
CJ

Even if method used in survey is not given, the argument is still backed by Federal Health Authority.
In that case , will B wont be right , as Great Taste AND great Health are tied up?

IMO, the correct answer should be B.

Experts have not evaluated the taste and hence concluding that if you want to have a great taste and health, go for the particular gum would be in correct. Please let us know what is the OA.