When a candidate gives a completely wrong answer, there are three main possibilities:
- The candidate doesn't know the answer (or learned false facts).
- The candidate misunderstood your question, but would otherwise have been able to answer correctly.
- You don't know the answer, or the question is complete bollocks to begin with.
Stating that an answer is wrong, and even moreso explaining what you expected to hear is in my opinion a mistake. There is nothing to gain from your point of view, but you may very well have disadvantages from doing so. If nothing else, you lose time and may discourage an otherwise acceptable candidate. Or, you will find the next candidate (who, in reality, is none better, maybe worse) being prepared to answer this exact question the way you expect.
A better approach would be to ask the candidate to explain his answer. You can learn a lot from the explanation even if the answer is still completely wrong.
You learn not just the amount of facts and trivia that the candidate has learned by heart, but you learn about his personality and mental ability, in particular the ability to reason, explain, and to adapt to situations.
Note that the ability to store facts and trivia is not the only, and not even the primary thing that makes a good candidate.
The candidate might explain well, or he might get nervous, he might feel offended for having his answer challenged. He might completely lose it, or he might tell you "Alright, you got me there!".
The candidate might give a wrong, but otherwise entirely reasonable answer. Or, he could give an explanation which justifies an answer that is, from a different point of view, perfectly correct -- only just not the one you had in mind. He might discover, and admit, that he has made a mistake half the way down.
This can tell you a lot about whether or not this person will be a good fit by personality, and ability/skill (not dry knowledge). Which is what matters more.
Of course, it doesn't hurt to have the candidate explain one or two correct answers as well. If for no other reason, then to not give out the clue that an answer was wrong.