I think this one is a real stinker. I agree that D is probably the right answer.
Although this is worded as an inference question, the stimulus (the paragraph) clearly provides a flawed argument, with a conclusion, evidence, and some missing assumptions. The vast majority of inference questions simply provide evidence in the stimulus, and ask you to choose the answer choice which is a logically sound conclusion based on the evidence. This one belongs to a rare and more difficult type.
In this rare type of inference question, what you must find is a NECESSARY assumption. The question asks for something that MUST be true if the statements in the paragraph are true. As noted already, the paragraph contains evidence and a conclusion which doesn't follow from that evidence. If that conclusion is nevertheless TRUE (as the question says), then any necessary assumptions must also be true. If those assumptions weren't true, then the conclusion would not be true.
The conclusion is that overall automobile costs will go up if the government introduces a new restriction. The evidence is that cars become safer if the government adds restrictions, and that safer cars can be sold at a higher price. For overall automobile costs to go up, these safer cars must actually BE sold, which is what choice D provides.
Choice B looks attractive as well, but it appears to be unnecessary. The evidence states that cars DO become safer if the government institutes restrictions, so B more or less restates this piece of evidence.
A question might arise because the conclusion refers to new safety features IN GENERAL, using side air bags as an example, while choice D (and A and B) refer specifically to side air bags. This still works: The conclusion makes a claim about ALL new safety features, and so if it is true, it must apply to ANY new safety feature -- such as side air bags.
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