1.I'm sure the answer is 我没在吃饭。 In Chinese grammar,"zài" + verb" is one of patterns indicating that an action is in progress.And the negative form of ,"zài" + verb" is made by placing "没" before the predicate verb.The pattern should be : "没+zài + verb" e.g.我没在看电视(I'm not watching TV) e.g.他刚才没在听我说话。(He was not listening to me just now.) note that,the progressive of an action may take place either in the past ,the present or the future.if the time is not clearly indicated or shown by the context,we may generally think the action takes place in the present. 2.FAQ about不 （bu）and 没(mei) Last week one of my students in class told me that her boyfriend wanted to buy an iphone4 last Sunday,but he hasn’t bought it because it’s too expensive.She said in Chinese“太贵了(tài guì le)，他不买(tā bù mǎi)”, and then I told her that in Chinese it should be “太贵了(tài guì le)，他没买(tā méi mǎi)”.So when can we speak不(bù) and when can we use 没(méi)? As most foreigners have known, 不 （bu）for 不是 (bu shi) or for 不（bu）+verb，没（mei）just only for 没有(meiyou). Actually,不 （bu）and 没(mei) are both used to negate, but they are used differently when we want to express some negative state of an action. Here are some difference between 不 （bu）and 没(mei). 不(bù) is used to negate actions at the moment or in the future.But 没(méi) is used to negate actions in the past.Please try to feel the distinction the following sentences. e.g.我(wǒ)不(bù)回(huí)家(jiā)。(I’m not going home.) 我(wǒ)没(méi)回(huí)家(jiā)。(I haven’t gone home. ) e.g.他(tā)不(bù)出(chū)差(chāi)。(He isn’t going for a business trip.) 他(tā)没(méi)出(chū)差(chāi)。(He hasn’t gone for a business trip.) e.g.他(tā)不(bù)去(qù)吃(chī)午(wǔ)饭(fàn)。(He isn’t going to have his lunch) 他(tā)没(méi)去(qù)吃(chī)午(wǔ)饭(fàn)。(He hasn’t gone to have his lunch.) Then I think it would be prefect to express themselves exactly in Chinese if foreigners can master this point.

不 indicates present or future actions. 没 indicates something that has not happened. Here are some examples: 明天不会下雪- It wont snow tomorrow(future), 今天不下雨- its not raining today.(present) it also goes along with verbs to express "not" 我不去图书馆-im not going to the library. 我不来上课-Im not coming to class. 我不在家- im not home. 他不是老师-he is not a teacher. also, it can be used to say that something "cannot" happen: 他不会说日语-he cannot speak Japanese. 我弟弟不会拿筷子- My little brother cannot use chopsticks. 他今天不能出来-he cannot come out today. 不can also be used for a rhetorical question: 你不是去图书馆了吗？- Didnt you go the library? 你不喜欢吃香蕉吗？-Dont you like to eat bananas? 她不是也要去吗？isnt she going too? 不 is even used to deny permission: 橡皮借我，可以吗？May I borrow an eraser? you can respond: 不行 or 不可以 "no you may not". 没 is used to say something has not happened. 我今天没吃早饭- I didnt have breakfast today. 我昨天没看电视。I didnt watch t.v. yesterday. 我上个星期没去公园－I didnt go to the park last week.我已经三天没吃东西了Its been three days since I've last eaten something. Note that 没cannot be used to for future actions. its used for something that has not happened whether it be today, yesterday, or even sometime before that 没 is also used to express you dont have something/there isnt anything. 我没有书包- I dont have a bookbag. 没什么奶茶- there isnt any milk tea. Something has never happened: 我没看过那部电影-I havent seen that movie. 我没去过中国-I haven't been to China. Finally, To say "I am eating" : 我在吃饭。 to negate it: 我不在吃饭。 没 cannot be used because it indicates actions that HAS NOT HAPPENED. Example: If somebody asks: Where have you gone today? you'd respond 我哪里都没去- I have not gone anywhere. If somebody asks 你去哪里？Where are you going? you would respond: 我哪里都不去-"I am not going anywhere." In the first situation the person asks about the past "where have you gone today?" so you cant use 不 and conversly, in the second situation, the asker asks "where are you going(now)?" so you cannot use 没 since you are talking about the present.

The Chinese seldom say: 我没在吃饭 although it's right. I think it's the reason why some people told you 我不在吃饭（it's not correct）but others told you 我没在吃饭。As a Chinese, if someone ask me: Are you eating? I wouldn't like to say:我没在吃饭， but just "没有 or 不是 or 没"， and most people would say:没有，not 我没在吃饭。 The following is common answer to the question:(Are you eating?) 1.没有 2.不是 3.没 4.（我）没吃饭（虽然这句话单独出现可能会有歧义，但是在具体的语言环境中还是很普遍的） 当然了，如果你确实想单独表达这个意思，而不是回答问题，你也可以说：我没在吃饭。

没 is a negative prefix for verbs in particular. 不 is also a negative prefix ,but not only for verbs. So you can never say 没好, but you use instead 不好 In your example I would say: 我不在吃饭 To negate this form 'zai + verb' I think using 'bu' is the correct choice. With other verbs you could only use 'mei' as in : 没有 我没有空。 When to use ‘没‘ or ‘不’ to negate a verb in general is not very clear to me . I think in most cases you will be using ' 不‘ . A Chinese native is better qualified to answer this question accurately.

我不在吃饭。 我没在吃饭。both are correct