Learn Farsi: مگر او ایرانی نیست؟ چرا، ایرانی است.

While مگر can mean 'unless', when placed at the beginning of a question, it shows surprise or disbelief. It works like saying 'Wait, is it not true that...?' or 'Don't tell me that...'. Here, the speaker expects him to be Iranian and is surprised by the possibility he might not be.

Why does the answer start with چرا? Doesn't that mean 'why'?

چرا usually means 'why', but it has a very important second job. Whenever someone asks you a negative question (like 'Isn't he...?' or 'Don't you...?'), you use چرا instead of بله to mean 'Yes'. It contradicts the negative question to say 'Yes, he actually is!'

Is it common to drop the word او (he) in the second half?

Yes! Because Farsi verbs and context usually make it obvious who we are talking about, native speakers often drop the subject pronouns. Since او was just mentioned in the question, repeating it in the answer would sound unnatural.

How would a native speaker pronounce ایرانی است in everyday conversation?

In casual spoken Farsi, the formal 'to be' verb است often shrinks down and attaches to the end of the previous word, sounding like an 'e'. So, ایرانی است is usually pronounced as ایرانیه.