i want to select a best answer but my question is not listed in my questions?

If the question is not location in your profile activity under My Questions then is was reported and removed. You should have received an appeal notifications that gives you the option to appeal the deletion. You can use the appeal form in the help menu in the spoke wheel. But it is hard if you don't know the word by word question or answer.

If its not in there then your Question may have been Deleted Due to a Violation of the Yahoo Answers Community Guidelines

After an hour, it should allow you to press "Best Answer". That's crazy !