The word "哪个" is formed by combining the interrogative character "哪", which itself is a fusion of the "mouth" radical (口) suggesting speech or question with the character "那" meaning "that", and the character "个", a common, neutral counter or measure word for general objects or individuals. Together, they structurally create a single unit meaning "which one", where "哪" introduces the selective question and "个" provides the singular, individualized unit being selected from a group.