『嗯。』 or 『嗯，吃了。』or 『吃了。』or

吃饭了没有 means Have u eaten or not? so u reply yes I have or No, not yet.in Chinese: 吃过了 or 还没呢。

吃了 or 吃过了 “.......过了” = have/had already done something

this kind of question should be answered by "yes" or "no". In this case, we say "verb + 了" meaning "yes" and "没有" meaning "no". "吃“ is the verb here, so you can say: 吃了 or 没有. "没有" is just like english --- "have not". Have you eaten? -- "no, I haven't". If you want to say: not yet, in chinese it's "还没有" or "还没"

You can say 吃了 . You can say 没吃or 没有, if you hadn't eaten.