Question: I've read in the Chinese Grammar Wiki, that if you want to express "which", you have to use the following sentence structure:

> 
> Subj. + Verb + 哪个 (+ Noun) ?
> 
> 
> 

But in the book HSK Standard Course 1, there is the following question:

> 
> 你是哪国人? (The book translates it to "Which country are you from?".)
> 
> 
> 

Why is in this case the word 个 omitted before 国?

Answer: 
Two clarifications, one about the structure, the other about the measure word.

**1) The structure**

> 
> Subj. + Verb + 哪个 (+ Noun) ?
> 
> 
> 

The above structure cannot be generalized and applied to all contexts. It's just an example. Another example structure can be:

> 
> 哪 (+ Noun) + Verb + Object?
> 
> 
> 

As you can see, in the above structure, the interrogative particle 哪+Noun is the subject of the sentence.

**2) The measure word**

The measure word 个, as well all other measure words, is not related to the interrogative particle 哪, but to the noun. I mean that you could also need to say, for example, 哪位, 哪些, 哪台, 哪本, etc. according to the measure word the following noun needs.

**你是哪国人?**

You could also say 你是哪个国家的人？But I would prefer the already commonly used shorten form 哪国人.
The difference of 哪 and "哪个" is that "哪" means "which", while "哪个" is shortened from "哪一个" that means "which one".

Below are two equivalent sentences but the grammatical arrangement:

你是 哪国 人? Or 你是 哪 国人? - Which country do you belong to/come from?

你是 哪个 国家的人? Or 你是 哪个 国家的 人? Which (one) country...? Note, "哪个" was shortened from "哪一个".

But note, there is a complete phrase after 哪个 instead of a simple noun. So,
it is incorrect to say: "你是 哪个 国人?".