"Whose coat is it?" should be accepted. "Whose is the coat" sounds a bit less natural to me as an English native.

Should be "whose coat is it?"

This sentence literally translates "Of whom is the coat?", which is grammatically correct but was marked incorrect. This translation is almost never said in conversational English anyway. However, the translation "Whose is the coat?" is NEVER said in English. "Whose coat is it?" should be the correct translation.

"Whose coat is it?" seems a better option for me as well