If the interviewer knows you have complaints about your current job, then they might ask you this question. Make sure the complaints are things that do not make you look bad. For example, having no opportunities is a good complaint, but not getting a good raise is a bad complaint. After you decide on the complaint, then you can create your answer around it. It is better to answer that you did bring this up multiple times. That way, you are displayed as honest.
Having an employee that complains about a lot of things is an employee you do not want to hire. However, if an employee complains about not having enough work, it could be seen as a good complaint. Most work places have more work than resources, so giving more work isn't a problem. So this complaint shows that you are a person who wants to work and wants to learn. These are good traits and that makes this answer a good one.
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