There are lots of ways your examiners could begin your viva, lots of questions to start the discussion.
It could be “how did you get interested in this topic?” or “how would you summarise your findings?”
Maybe they’ll ask “what’s your most important result?” or “why did you decide to follow this line of enquiry?”
Or maybe they’ll simple ask “how are you feeling today?”
There are no trick questions in the viva, especially with whatever your examiners ask first. The first question is likely to be something you’ve thought about before; you can’t guarantee what it will be exactly, but you can be sure it’s something you can answer.