Final answer:
The correct interrogative word to complete the question is 'Quiénes'.
Step-by-step explanation:
The correct interrogative word to complete the question is Quiénes. The question is asking about the identity of the group of people referred to as 'ellos', so we use the word 'Quiénes' which translates to 'Who' in English. The correct question would be '¿Quiénes son ellos?'
Learn more about Interrogative words in Spanish