How do I chose a 'best answer'?

For yours:

Wait a minimum of 4 hours and the buttons to pick a Best Answers will show up. Select a Best Answer before 4 days and earn 3 points. After 4 days, if you don't, the question 'expires' and goes to voting.

For others:

Click the VOTE tab on the far right of any section. Pick a question, vote on an answer. Get 1 point.

The only other question you have listed shows that it is less than 1 hour old. So it is too soon to choose a Best Answer.

In order to choose a best answer, first of all you may want to consider reading all of the answers you got thoroughly and then decide which one is the best.

Then, just go back to your question and you will see the blue buttons of “Choose as Best Answer” next to each answerer. Pick the very one you think it’s the best, comment it and rate it and then it’s done.

In case you don't know how to get back to your own question, then just click on your photo (avatar) or your name and you will go to your profile page. Then scroll down and you will see there is a section with 4 tabs: “My Answers”, “My Questions”, “My Starred Questions” and “My Watchlist”. Click on the tab of “My Questions” and then click on the question you want to choose the Best Answer for.

But remember, you need to wait for at least 4 hours before choosing the Best Answer and if you haven’t chosen the Best Answer, the question you asked will automatically turn into “Undecided Question” and then it has to be voted by other Yahoo! Answers’ users to pick up the Best Answer for you.

Hope this helps.