Here's a breakdown of the consultant's argument: The mayor shouldn't do X. If the idea works, something bad may happen. If the idea fails, something bad will happen.
Answer (B) breaks down to: Joni shouldn't do X. It's unlikely that something good will happen, and if it works something bad will happen.
Answer (D) breaks down to: Alvin shouldn't do X. If it works, something bad may happen. If it doesn't work, something bad will happen.
When you break down the arguments into more abstract statements it becomes clear that (B) does not follow the exact same structure as the argument while (D) does. The key is to describe the arguments at a high level of abstraction, while taking care to notice different verbs and qualifiers, as that will let you see the common themes.