The argument says, in part, "let's reduce pollution by replacing planes with trains". That's not a good proposal if trains pollute more than planes, so B is clearly one good answer here.

But this question shares a problematic feature with many prep company questions: it's not clear which of the proposal's goals is most important, nor on what basis its opponents are objecting to it. The argument justifies its proposal because it would "cost far less" than airport expansion, because it would relieve "airport congestion", and because it would reduce air pollution. But if answer D is true, and many new airports are being built in areas currently without trains (which is most areas, since as the stem tells us, trains connecting major cities don't exist right now), that suggests an alternative: do nothing, The new airports alone might relieve the airport congestion, and it clearly costs less to only build those airports, which according to D is happening no matter what else is done, than to both build the airports and either build trains or expand existing airports. So if the goal is to be "cost-effective", D suggests a reason not to go ahead with the train proposal.

I'd still consider B the 'best' answer to the question, but D is too good an answer to be wrong on a real GMAT CR question. In real GMAT CR questions that propose a policy, there is usually one clear goal of the policy, and not three, which makes it easier to tell what kind of answer you're looking for.