The word "哪个" is formed by combining the character "哪", an interrogative meaning "which", constructed from the radical "口" (mouth, often indicating speech or questions) attached to "那" (that), with the character "个", a generic measure word for objects or people. Together, they function as a single unit meaning "which one", where "哪" provides the interrogative force and "个" acts as the counter for the item in question.