An answer is a kind of response. An answer is grounded in truth or a compelling argument. An answer could be what you offer after a question…
…if the question is part of a quiz. But the viva isn’t a quiz. It’s not an interview. It’s not even a question-and-answer session.
The viva is a discussion, steered by the questions of your examiners and the responses you offer. A response could be an answer depending on the question – but it could also be an opinion you offer, an idea that you share, a question to clarify a point or a hunch that you feel. There’s a place for answers in the viva, but you might not have an answer for every question.
However, given your knowledge, your skills, your work and your experience, it’s reasonable to expect that you could respond to every question.