Consider this. Your child chose C because it is his job. His job to practice being in charge of himself as often as possible. Her job to test you, to let you know HER preference, to state loud and clear “I am the boss of ME!” And your child is right. He IS the boss of himself, and as the boss, he gets to ultimately decide what choice he will make. This is truly evidence of just the kind of self-directed, independent soul you (most of the time) want to grow. Someone who is in charge of themselves.
Okay, but you still need to get out the door. To continue to support your child in their quest to be independent it is important to respect their choice. How does this look and still get out the door–maybe on time?