If you are asking about 'which expression is correct?' I rather can give you an answer that both '무엇이에요' and '뭐예요' are correct words(actually, '뭐예요' is more common word to use), but you need a subject to complete sentences. for example, 뭐에요? -> what? 이것은 뭐에요? -> what is this? I hope this answer can help you to understand Korean more. thanks!