howto set a category for a webform/entityform question ...?
You can do this with the Webform Entity module. It uses Webform to create the forms but the content that is created is a full-fledged entity instead of just a serialized array in the database. That means full Views support etc.

For a category you can use a field list in the webform which can become a taxonomy field or field list in the entity that can be used as an exposed filter in Views.

To set this up just go to "admin/config/content/webform/entities/webform/" and click "Create" to create an entity webform



Then add fields to the entity:


Now, when you create a webform, the fields from the webform entity will appear on your node:



Finally, in Views, create a new view with the webform subsmissions as a base table:



and the fields attached to webforms can be used as exposed filters, fields and all other things views: