ရှင်းပြချက်
This is a question-word question with 哪 (nǎ, which): 哪国人 means “a person from which country”. The safest answer keeps the question and replaces only the question word: 你是哪国人?→ 我是越南人。 Don't just say “越南” — a one-word reply has the right meaning but loses points because it is not a sentence. If you have time left, add one short, safe sentence such as 我家在河内. Watch the sounds: 是 (shì) is retroflex, 人 (rén) rises and ends in -n, and 越南 (Yuènán) is said in Chinese, not as “Việt Nam”.