Seeking a competitive advantage, some professional service firms (for

5. Which of the following hypothetical situations best exemplifies the potential problem noted in the second sentence of the second paragraph (lines 14-17)?

(A) A physician's unconditional guarantee of satisfaction encourages patients to sue for malpractice if they are unhappy with the treatment they receive.
(B) A lawyer's unconditional guarantee of satisfaction makes clients suspect that the lawyer needs to find new clients quickly to increase the firm's income.
(C) A business consultant's unconditional guarantee of satisfaction is undermined when the consultant fails to provide all of the services that are promised.
(D) An architect's unconditional guarantee of satisfaction makes clients wonder how often the architect's buildings fail to please clients.
(E) An accountant's unconditional guarantee of satisfaction leads clients to believe that tax returns prepared by the accountant are certain to be accurate.

This is what the relevant lines say:
With its implication that failure is possible, the guarantee may, paradoxically, cause clients to doubt the service firm's ability to deliver the promised level of service.

What does this mean? It means that if a business claims, say, "full refund in case of failure", it causes clients to worry that the business is saying that failure is a possible case.

Option (D) exemplifies this:
(D) An architect's unconditional guarantee of satisfaction makes clients wonder how often the architect's buildings fail to please clients.

but option (B) doesn't
(B) A lawyer's unconditional guarantee of satisfaction makes clients suspect that the lawyer needs to find new clients quickly to increase the firm's income.
To exemplify, it needed to be something like this:
(B) A lawyer's unconditional guarantee of satisfaction makes clients suspect that the lawyer might be losing some cases.