You are prompted at the moment a ticket reaches a closure status. There are three main paths:
Reply and resolve/close - from the Reply tab, choosing "Submit and set as Resolved" or "Submit and set as Closed" lets you pick the Root Cause in the same step, without leaving the reply screen.
Create a ticket as Closed - raising a new ticket and using "Submit as closed" prompts you to select a Root Cause before the ticket is created.
Change status from the ticket list or ticket details - moving a ticket to Resolved or Closed prompts for a Root Cause.
Customer experiences are managed better with Raiseaticket.