Question

? Complete the question with the correct question word: ¿_______ es la hermana de Marcos?
1.Dónde
2.Cuáles
3.Quién
4.Qué

Answer 1

The answer is 4=Que  because that means who in Spanish.
(so your sentence is who is the sister or Marco)

Answer 2

The answer would be 3. Quién, because "quién" means "who" and the question is asking "___ is Marco's sister?" So, "who" or "quién" would go in the blank to make the sentence "Who is Marco's sister?"