Answer:
Quién es.
Step-by-step explanation:
Remember that when you are faced with language problems you have to read the full context in order to choose the most appropiate words, in thsi case Nina is asking _________ your best friend? and Carlos answers She is Alejandra Ríos, so we can assume that Nina is asking Who is...? so the best translation for that in spanish would be Quién es...?, so that would be the best option.