Learn Farsi: این جواب غلط است.

Because غلط (wrong) is the predicate of the sentence, not an adjective directly attached to the noun. You are saying "This answer IS wrong," where این جواب is the complete subject. If you wanted to say "this wrong answer" as a single noun phrase, you would then use the ezâfe: این جوابِ غلط.

How would this sentence be pronounced in everyday spoken Farsi?

In spoken Farsi, the formal verb است (is) is usually shortened to an 'e' sound attached to the end of the previous word. So, غلط است becomes غلطه (ghalate). The whole sentence would be spoken as این جواب غلطه.

Could I use اشتباه (eshtebah) instead of غلط (ghalat) here?

Yes! Both mean "wrong" or "incorrect." غلط often implies something is factually incorrect or breaks a specific rule (like a math answer), while اشتباه can also mean a general mistake, error, or misunderstanding. For grading an answer, both work perfectly well.

Is جواب a formal or informal word for "answer"?

جواب is the most common, everyday word for "answer" in both spoken and written Farsi. If you are reading a highly formal text or taking an official exam, you might also see the native Persian equivalent پاسخ (pâsokh) used instead.