Hi Shravani,
To moved a discussion to "Closed" currently it can be only done by admin. If the post you created a Question type (instead of Discussion):
On each the comment you should see a:
https://us.v-cdn.net/5019921/uploads/editor/9e/3f6ga4q8t6b8.png" width="976">
If comment solved the issues for you, click yes and the thread will be shown with a green "Accepted Answer".
For the "gray answer" tag - Currently it is a little mislead and are working to get it changed. The gray answer tag implied that the Question has been commented on and is awaiting the original poster of the thread to accept or reject the comment as being the answer.
If you have a thread that doesn't have the "Did this answer the questions?", it meant that it is a discussion type. If you would like any thread converted over to "Question" type post, please let me know the post and I can convert it over.