How to tackle the questions which you don't know in an interview?

It is true that we all dont know 100% about every topics, what is important is to that our preparation should be there, before going for a interview always be prepared for the questions which could come up according to the job domain.
For example if the Job profile is of digital marketing intern be familiar with topics such as SEO so that you have a general idea and you can be confident about them while answering in interview.
If you dont know about a question dont blabber around because the interviewer already know the answer for the question, if you know something about the question answer that and say i know this much rest i dont know or i am not familiar with the topic.