To answer ¿De quién es? , you cannot just say "Juan." You need a possessive pronoun that agrees in gender and number with the object owned.
"¿De quién es la maleta?" (Whose suitcase is it?) Answer: "Es de la profesora." (It's the teacher's.) p219 estructura 1 de quien es practice it hot
In English, we usually show possession by adding an apostrophe and an "s" (e.g., book, John's car). Spanish does not use an apostrophe. Instead, Spanish uses a prepositional phrase involving the word de (of). To answer ¿De quién es