Should I down-vote the question because the accepted answer is not a good one?
Even though Stack Overflow has aspects of a wiki, original authors are given a certain amount of authority to select the best answer for their particular situation.
The community then selects, through voting, the best answer according to their own reckoning.
The question asker should not be penalized because it turns out that the most helpful answer to them does not match the most helpful answer as chosen by the community. This is an example of the system working very, very well. You'll note that there are many questions where the chosen answer was not the highest voted.
If you disagree strenuously with the author, then you should post a comment with your reasoning. Otherwise, it's best to let the system take care of things. People who come along later from a Google search with a similar question will take the time to read at lest the first few answers, and therefore will get all the highest rated information regardless of what is chosen.
If this is a complaint because "rep was given where it wasn't due" then you are simply out of luck. They posted the question, they can give the answer bonus.
Stack Overflow was designed to rate questions and answers, not users. Penalizing the question isn't going to accomplish anything. Open a dialog with a comment if you want to change behavior.
Regarding this specific instance, the answer was selected because the author of the answer pointed out that the question is no longer applicable - since there is only one option, then there is no ambiguity.
Further, the highest voted answer appears to be nothing more than a guess. He's giving what he would expect to happen, but there's no veracity or authority to his statements.
I believe in this case the selection of answer is perfectly reasonable.