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First, a practical point. Companies set their own eligibility rules for campus and off-campus hiring, and some have conditions about backlogs or gaps. These differ widely and change from year to year. Read the criteria for each drive, ask your placement cell if anything is unclear, and do not misstate your record to meet a rule. Background checks after an offer often look at the same documents.
When it does come up in the interview, answer in three short parts.
What happened, in one or two sentences. 'I had a backlog in second-year mathematics. I cleared it in the next attempt.' Or: 'After graduating I spent a year at home because of a family situation.' You do not owe the interviewer every detail. A personal or health reason can stay general: 'for family reasons' is enough. Interviewers are used to this and most will not press.
What you did about it. This is the part that matters. 'I changed how I prepared for exams, and my marks after that were steadier.' 'During that year I did an online course in data analysis and helped with my uncle's shop accounts.' Be specific. If you did very little in a gap year, say what you did do and do not inflate it. A small, true thing is better than a large, vague one.
Where things stand now. 'All papers are cleared and I have no pending exams.' 'I am ready to join and relocate.' End here, so the conversation moves on.
The whole answer should take under a minute. The common mistake is to over-explain: a long story about a difficult professor or a bad semester. It makes the issue sound bigger than it is and keeps the interviewer focused on the past.
Practise it aloud until it feels routine. A nervous, hesitant answer suggests there is more to the story. A calm, short one tells the interviewer you have dealt with it, which is usually what they wanted to know.
If an interviewer reacts badly anyway, stay polite and do not argue. Not every company will be a fit, and your placement cell can help you find drives whose criteria match your record.