Every student with a gap year in their resume should be prepared for the question - ‘Why did you take a gap year?’, during your interview. The interview panelists are genuinely interested to know how you made the most of your break. You should be honest while replying to this question. You can simply answer that you needed more time for preparation.
During your break year, you can do volunteer work during weekends or learn something new. If you opted out of campus placements, you can do research projects or internships with your professor. You can do some certification or take examinations, in your field of interest, such as CFA/FRM. These will help showcase that you have spent your time productively.