How do you choose a specific detaile yahoo answers category like before by smartphones?

Tap the 3 horizontal bars at the top left of the page (to the left of the yahoo! answers logo). You should see the category list below "Ask a question", "Send Feedback", and "Notifications" (or at least that's how it works when using Safari browser).

you cant before the question is posted after its posted you can pick it before you post it yahoo will try and guess what category to place it in based on key words used in your question if its not placed correctly you can move it to move it go to your question under the blue edit menu click on edit category then select the new category to put it in then click submit