what is the use object slicing

First of all, it's quite useless to prepare interview answers. Please see my past answer: SSRS interview questions and answers[^].

But I would understand that it's just your curiosity. It would be a great thing if you paid any reasonable effort to learn what it is. It would be so simple:
https://en.wikipedia.org/wiki/Object_slicing[^].

But if you did not answer this question on the interview, you should not be too frustrated. I would say, this question is quite bad, maybe it even could be called an idiotic question. It does not reveal any understanding, knowledge of theory, nothing. It's just about knowing one quite rarely used and not particularly useful term, something which no one should be required to know, unless this is a part of some specific internal corporate culture (which you also would not supposed to know). I mean, many people who understand OOP to the finest detail may also be unaware of this limited-use form. The meaning of the term is just nothing special, something pretty trivial and hence hard to remember, which does not really needs a special term. People experience in OOD face enormous number of aspects of the same level which never got a special term for that, and don't suffer too much. :-)

So, practically, during the interviews, you can try to block the attempts to get you in awkward position by telling something like "if you tell me what do you mean by this term, it's likely that I understand well how to work with such things, because I'm quite competent in OOP and OOD". Of course, you should really be confident in the matters.

—SA