¿_______ <em>es la hermana de Marcos</em>?
Translation:
[Blank] is Marcos' sister?
<u>The options</u>
A. <em>Dónde</em> - where
B. <em>Cuáles</em> - which ones
C. <em>Quién</em> - who
D. <em>Qué</em> - what
Option B can be eliminated because it is plural, while the verb (<em>is</em>) is singular.
Option D can safely be eliminated because it doesn't make sense to ask "what is his sister?" in this manner under any circumstances.
It might seem like option A, <em>dónde, </em>is the correct answer, however, when asking where someone is, <em>estar</em> is used, rather than <em>ser</em>.
You would say <em>¿Dónde esta la hermana de Marcos?</em>
The leaves option C, which is correct. The question is <em>Who is Marcos' sister?</em>
Answer:
C. <em>Quién</em>
