Final answer:
The most crucial time to flash your brake light is when you anticipate that you will be slowing down suddenly. This warning is key to preventing rear-end collisions by alerting drivers behind you to react accordingly.
Step-by-step explanation:
It is MOST important to flash your brake light when you will be slowing suddenly. This action ensures that drivers behind you are aware that you are about to decelerate quickly, reducing the risk of a rear-end collision. Situations that might require sudden slowing could include unexpected traffic congestion, road hazards, or if the car ahead of you is stopping abruptly.
While the other options provided may also be situations where caution is needed, they do not usually require the urgent notice that a sudden slow down does. For example, if someone is following too closely, maintaining a steady speed and gradually slowing down gives them time to react. A stop sign ahead is a predictable event where following drivers expect you to stop. If your signals are not working, it's critical to repair them as soon as possible, but using your brake light would not be a substitute for turn signals.