C does accurately describe something that is true of the argument, but I'm not sure it's a flaw. The argument is that since the power plant won over its biggest critic, we should ok the project. The issue of whether the critic is a moron isn't really relevant to the argument as such. If we win over our biggest critic, even if they are a moron, we surely have helped our case.
More so, I think C might be going to far. We don't need to know that outspoken opposition is NECESSARILY informed, since we are only dealing with one specific case.