We are looking for an answer choice that makes the same error as the passage here.
In the passage there is discussion of a sports car with a high-efficiency (good on gas) engine vs. its regular model. We find out it would take 60,000 miles at current fuel prices for this high-efficiency car to even out in cost with the regular model. The passage then wraps up by saying, that if fuel prices fell, it would take a smaller number of miles to even out price wise.
But the whole point of the high efficiency engine was to save on gas. If gas is even cheaper in this hypothetical, that wouldn't result in less miles to reach break even. It would result in more!
Answer choice C does the same thing. This Roadmaker saves on worker wages. So how would having cheaper working wages make the Roadmaker even more advantageous? Higher wages would do that, but not lower.
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further.