In retail chains where store policy includes an allowance

I disagree with @carcass.
The correct answer choice is C.
This is how I approach bold face CR questions:

First! Analize the structure of the argument. Identify the premise and the conclusion.
Premise

In retail chains where store policy includes an allowance for merchandise returns based on dissatisfaction with the products, customers are twice as likely to report that they are dissatisfied with products as they are in retail chains that do not have such a policy. - an evidence, a trend in the customer behavior
Although there is no objective test for customer satisfaction -- no way to determine whether a customer is telling the truth

Conclusion
it is not logical to conclude, as some have done, that half of all these claims of dissatisfaction are false. main conclusion

because in retail chains lacking a satisfaction policy for returns, there is often no incentive for customers to report real dissatisfaction in the first place. reason supporting the main conclusion .

Now, we can see that the second bold face supports the main conclusion. As a result we can eliminate A, D and E.
Finally, between B and C. The second bold face is a reason that the claim might not be true. By no means the second bold face is supporting the accuracy of the claim. Therefore the answer is C.