Need description of Error

The error in reasoning with this question is that there is no evidence to support the conclusion. We do not know why Mayor Smith originally opposed the construction of the nuclear power plant. We also do not know why Mayor Smith has now changed her mind. What if she changed her mind because of the personal benefits she would receive if a nuclear power plant was constructed? Then it wouldn't be true that "there is good reason to believe that [the construction of the nuclear power plant] will be safe and therefore should be built." We have no information about safety. Answer choice (E) clearly points out how the argument does not give any evidence or support to its claim. Hope that helps! Let me know if you have any other questions.

why not D?

Answer D is incorrect because what we are after is the reason the argument is vulnerable to criticism. Answer D gives us an additional conclusion: that any project favored by the Mayor should be sanctioned on the basis of her having spoken out on it. But it does not tell us anything about the faulty logic linking the premise and the (different) conclusion in the passage. I hope this helps. Please let us know if you have further questions.