What would you do, if you find someone's opinion or answer incorrect?

I will never  vote down, voting down will give him negative points, what  am I going to get from doing that ?

I will just express my opinion underneath his answer,  and oppose to  him  in a polite way,

so n°2

2.
I'll express my opinion

Firstly opinion differs from answer be it correct or incorrect.
As long as it is opinion it must be respected.
The moment I discover if someone is incorrect and if the nature of circumstances makes it mandatory to correct it right on spot then I would gladly go for it.
Other scenario is if its someting that can be corrected later for reasons like risk of respect of the person who is incorrect can be compromised in front of others or if the person is extremely angry and is not in a state of mood to accept the correction then you must wait for the right moment and then gently approach, remind and correct it in a respectable manner.

What I try to do is actually to challenge the person asking many questions that will raise the doubt in his/her wrong opinion answer, and then advise/ share my feedback with the right answer.
This approach will not underestimate the person, and will guide him on how to analyze , think & respond properly.

Well, it depends on subject, environment, career, culture and ....etc; but let me consider that the case has happened among team members in one company, if so; I would implement the following:
1- Explore my opinion in polite.

2- Explain the negative conclusions of his/her opinion.

3- Advise the appropriate ideal opinion for the case and accept the reasonable, logical and practical one, which serves our enterprise benefits.
Finally, we have to maintain the atmosphere to be quite and cool for the benefit of all parties; because as human being, there will be a day that I might have incorrect opinion on such case, so I am expecting that people should not be aggressive on their comments and reply towards myself :)

i will express my opinion but also handle it with care because some people don't like to be corrected and i will help to understand that person with proof the correct answer.
The main focus is also not to try and override a person but rather walk with that person to understand and except any person can be wrong in our life and handle it with care and professionalism.
But also make them feel good for at least trying because sometimes something wrong can make that person negative.

2.
I'll express my opinion.
If it's required.

Immediate correction without preying dignity or ridicule him

(B) I will express my opinion and politely highlight the mistake in his answer. Will never vote him down as this might discourage him to participate further and it will result in demoralizing the individual.

2.
I will express my opinion.

2.
I will express my opinion.
I might even clarify in my answer why I believe that this answer is wrong.
At the end exploring different opinions is the only method to reach to a more comprehensive idea about the topic and the reader has the full right to agree to the one which sound more logic and accurate