Answer choice A is incorrect because the statement "transportation will be much more expensive on semiplaning monohulls than on conventional ships" is not an analogy. In fact, this statement on its own does not relate to jet airplanes at all. It also does not serve to support the main conclusion, that semiplaning monohulls will probably be profitable because it only gives a downside.
Answer choice D, on the other hand, does correctly describe the role of the statement under question. The statement brings about a counterpoint to the conclusion, but is then itself countered when the analogy between semiplaning monohulls and jet airplanes is made.