Fill in the blank with the most appropriate word.
What _______ your parents going to do when they retire?
A verb is used to express an occurrence, action, or state. It is the grammatical centre of the predicate in a sentence.
An auxiliary verb is a verb which gives is used in the formation of tenses, moods, tones, etc.
Option C is correct as the subject is plural and going to do when indicates the use of helping verb are.
The other options are wrong as the subject is not singular and the question doesn't talk of past.
The correct answer is C)are