Xin chào, Mina. Allow me to add other explanation. 个 is merely a measure word to denote specific thing that is mentioned. 哪 + 个 + 名词 (noun) The noun is usually general things. There are many measure words for specific noun (as in Vietnamese, I guess). For instance: 1. 你喜欢哪个颜色? Which colour do you like? 2. 他们上哪个班? Which class do they attend? If you know other measure words, you can ask question in the same way. e.g.: 1. 请问， 你是哪位? Pardon me, whom I talking with? 2. 她买哪件裙子? Which skirt did she buy? Chúc may mắn, 玲玲

哪 its can meaning where or which : 1) 哪里 = where (the place) 2) 哪个 = which (one) 个 its means ones So 哪个 in your sentence is mean which (one) class. Hope that helps!