I don’t think there’s ever a situation in the viva where you can do nothing.
I can’t think of a question that could be asked that a candidate couldn’t respond to. Questions from examiners are asked to prompt discussion. They’re asked to clarify what something means. They’re asked to dig deeper into your thesis and research.
By design, they’re not asked to crush you, stump you or stop you. They’re not asked to tear your work apart or make you feel small. They could be uncomfortable, they could be critical or they could ask about something that you’ve not thought about before.
You always have options for how you engage with them.
You just need to pause. Then you think. And you respond. A difficult question or comment cannot derail your success so long as you pause, think and respond.
There’s always something you can do.