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).
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.
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,