The engineer argues that wide open roads encourage more risk taking. By analogy, CO2 emitters/ drivers are currently deterred by the threat of global warming, should this obstacle/ obstruction be removed, they are more likely to engage in risky behavior and pollute more. (A) correctly summarizes the principle saying that conditions that create the feeling of security, e.g. removing obstruction from the road or the threat of global warming, only encourage risky behavior. There are a couple of issues with (D) - first, it is too narrow of an interpretation, the fact that "a technical fix" is used to reduce the threat of global warming is irrelevant, any solution would have the same impact; second, the argument suggests that a fix encourages risky behavior rather than "cannot discourage," "cannot discourage" would suggest that fix may have no impact, and risky behavior would occur regardless of whether the fix is implemented or not, whereas the argument tells us that the fix is the cause of the risky behavior.
Let me know if this makes sense and if you have any further questions.