It is MOST important to flash your brake light when:

Flashing your brake light is most crucial when you will be slowing suddenly because it serves as a vital communication tool to alert other drivers about your intention to decelerate quickly. Motorcycles can stop more quickly than many vehicles, and sudden braking can catch drivers off guard, especially those following closely. By flashing your brake light, you provide a visual cue that helps ensure other riders or vehicles have a warning, thereby reducing the risk of a collision.

While the other scenarios may also warrant caution and attention, they do not carry the same immediacy as the need for sudden braking. For instance, when approaching a stop sign, slowing down is generally expected and the notification might not be as critical as in unpredictable situations. Similarly, if signals are not working, it is important to find alternative ways to communicate intentions, but flashing the brake light is specifically about alerting others to your immediate actions, making option B the most critical scenario for using the brake light visually.