1. Click 
next to the Select a forum field to open the Select a forum form in the Discussion tab.
2. Click a specific category/forum title to select it.
3. Click Save to accept saving changes in the Discussion tab.
Select the category scoping
In the Category Scoping tab, show/hide categories in Answers. The hidden category is not displayed in Answers.
To hide/show any category, simply deselect/select its checkbox and click Save.
See also