Translation:Whose apples are these? These are my apples.
"Whose apples are these?" -> the subject is "these", "these" can be interchanged with "they" ("Whose apples are they?"), "these" is a pronoun that plays a noun role. An answer to this question: "These are my apples".
"Whose are these apples?" -> the subject is "these apples", "these" cannot be replaced by "they" ("Whose are they apples?"), "these" is an attribute of "apples", a pronoun that plays an adjective role. An answer to this question: "These apples are mine".
It just happens so that in English both roles are played by "this/these", and in Ukrainian the noun role is "це", and the adjective roles are "цей/ця/це". It's the same in Spanish, "esto" plays the noun role ("Esto es mi gato ...") and "este/esta" the adjective roles ("Este gato/Esta gata e mío/mía")
Please provide the answer you entered. Sometimes even when you think it's exactly the same, it's not; or it might be some bug or mistake on our side, but to fix it we need your exact input. I can check the acceptable sentences in the exercise, add new ones and correct the old ones, but I don't know what to correct from your comment.