What is the right course of action here?

In this scenario, the blue car faces a stop sign, which means it must yield and stop at the stop line. The correct actions here are:

1. Allow the blue car to pass through: The blue car must stop at the stop line and yield to traffic.
2. Stop on the stop line: This is a key requirement under German traffic rules when approaching a stop sign.

Not allowing the red car to pass through is essential because the blue car's obligation to stop comes first. Understanding this ensures that the flow of traffic is maintained safely and any potential accidents are avoided. Always prioritize yielding to vehicles or pedestrians that have the right of way according to traffic signs.