The most appropriate response is: A plot device increases tension.
This is because the scenario establishes unresolved conflict between the two characters, and the introduction of the driver of the red car who has a lighthearted interaction adds to the tension. The juxtaposition of the carefree scene in the red car and the ongoing argument between the two characters intensifies the emotional stakes and the unresolved issues between them, making the situation more charged as the driver approaches.