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Start by understanding why the question was asked. In most fresher interviews, the interviewer begins with something easy and keeps going deeper. Each answer decides the next question. At some point, the question will pass beyond what you know. That is where the interviewer learns the thing they came to find out: how you behave when you are stuck.
The first thing to do is stay in the conversation. Many students go silent, look at the table, or say 'sorry, I forgot' and wait for the next question. Silence tells the interviewer nothing about your thinking. Try repeating the question in your own words, which gives you a few seconds and checks you have understood it. Then say what you do know that is close to it.
Say plainly when you do not know. 'I am not sure about that part' is a perfectly good sentence. Then continue: 'But I know that it works like this, and if I had to guess, I would expect it to behave in this way, because of that.' That second half is the valuable part. The interviewer can now see how you reason from a known point towards an unknown one.
Label your guesses. A guess offered as a guess is useful. A guess offered as a fact is a risk, because if it is wrong, the interviewer begins to wonder about everything else you said. Students often feel that admitting a gap will lower their marks. In practice, a confident wrong answer usually does more harm than an honest gap.
Use a small example. If you are asked how something works and you are not certain, try a tiny case. For a data structure question, draw two or three elements and walk through what happens. For a circuit, a process or a formula, take a simple number or a simple case. Working through an example aloud is often how you find the answer, and even if you do not, you have shown a method.
Take hints well. Interviewers often give a nudge when a student is stuck. Hear it, use it, and say what it makes you think of. Do not argue, and do not say 'yes, I was about to say that' if you were not. A student who can pick up a hint and run with it looks like someone who will learn quickly on the job, which is exactly what a company hiring freshers wants.
Do not try to steer the interview away from the gap by changing the subject. Interviewers notice. If you really cannot go further, say so, and let the interviewer decide where to go next. Then let it go. A single missed question rarely decides an interview, but carrying it with you into the next three answers can.
Finally, prepare for it before the day. Ask a friend or senior to interview you and to keep asking until you cannot answer. Practise the sentence, practise the pause, and practise reasoning aloud. A mock interview is a good place to do this because nothing depends on it. Students who have practised being stuck tend to find the real thing much less frightening.