Currently you can only flag a topic not a single post. To change the status of the topic Simply edit any message inside the topic.

2) So what if user 1 post a question

if there are 10 answers/posts how anyone will figure out that which post has the answer ?

Also, if the user who originally created the topic forget to mark the question as solved or answered then the user who answered the question in the topic can't mark it ?

3) Is there any voting system for posts ? like stack over or other forums / communities these days ?

1. It needs to be enabled in the Host Settings under Features -> Allow Topic Status Option

2. thats currently not possible to flag a message as answer

3. No, only the thanks system