The correct option is C Whose
The raincoat belongs to a person as mentioned in the question, we need to place a possessive adjective in the blank, they denote possession of the noun. When we want to ask who the raincoat belongs to, we simply use 'whose?', option C is hence correct. The given question asks about the person whose raincoat is hanging, and not the raincoat itself, 'which' when placed in the blank asks about the raincoat and this changes the meaning of the sentence. Hence option A is incorrect. 'Whom?' is a question word that asks about a person, whereas the question word required here asks about the person who possesses the noun 'raincoat', hence option B is incorrect. 'What?' is a question word that inquires about quality or asks for information about something, whereas the adjective required here enquires about possession. 'What' refers to a thing or quality, and not a person, hence option D is incorrect.