Switching help!!

I have a friend who wants to switch using my link (ace!) But she has smart meters fitted at the moment, she wants to switch but on the first few pages it asks if she tops up using a key or card or direct debit. Which one should she pick? Thanks!!

on the first few pages it asks if she tops up using a key or card or direct debit. Which one should she pick?

Well, which one does she do? Does she top up a prepayment meter, or does she pay monthly by direct debit?