"Whose apples are these? These are my apples."
Translation:Чиї це яблука? Це мої яблука.
That is also correct... to some extent.
You see, particle це (singular, an equivalent to 'this/it') is what we use more often for such phrases. Unlike in English, in Ukrainian there is no difference between "these are" and "this/it is". Both correspond to "це (є)". And that is exactly our case when we need exactly this translation, because real subject here is "це".
From the other hand one may want to use a demonstrative (цей, ця, це — this; ці — these) instead of particle for emphasizing apples as the subject. With ці the question will mean something like 'These apples — whose are they?' Please compare:
Whose apples are these? — These are my apples. Чиї це яблука? (literally: чиї яблука є це) — Це (є) мої яблука.
Whose are these apples? — These apples are mine. Чиї (є) ці яблука? — Ці яблука (є) мої.