'Will' or 'Going to'?
The construction "going to" is very common in those situations where "about to" can also be used.
The car's about to crash! --->It's going to crash! (YES)
It will crash (YES but it has slightly less urgency.)I'm about to announce the winner ---> I'm going to announce the winner. (YES)
I will announce the winner (YES but it has slightly less urgency)We arrived just as the movie was about to start ---> … just as the movie was going to start (YES)
We arrived just as the movie [was] will start. (NO/UNGRAMMATICAL)
This usage to express urgency or say something is imminent, is common both in speech and in writing; the author of the exercise probably wanted to see if the reader/learner had understood this lesson.