The best answer is D. The passage states that “professional service firms with outstanding reputations and performance to match have little to gain from offering unconditional guarantees.” If a firm has an outstanding reputation based on a high level of performance, it is likely that its clients are satisfied with the quality of service that is delivered.
Choice (A) is not implied: the passage indicates that the author believes that firms with outstanding reputations have little to gain from offering unconditional guarantees, but the passage implies nothing about whether such firms either currently offer guarantees or have offered them in the past. The passage does not provide information concerning the profitability (B). In addition, it does not suggest that such firms practice in fields in which outcomes are predictable (C).