When is it appropriate to use your four-way flashers?

Answer: B — When slowing down unexpectedly.

Using your four-way flashers is appropriate in situations where you need to alert other drivers to a potential hazard. In the case of slowing down unexpectedly, activating your flashers communicates to vehicles behind you that there is a reason for your sudden change in speed, helping to prevent collisions and maintain safety on the road. When parked on a busy street, while it may seem reasonable to use flashers to signal your presence, the primary intent of four-way flashers is to warn of a hazard, making it less suitable than the unexpected slowing scenario. If your vehicle runs out of fuel, you would typically pull over and may use flashers, but that is not a proactive warning about a decreasing speed. Changing lanes usually requires signaling with turn indicators, as it's a normal driving action rather than an indication of a hazard requiring all four lights. Thus, using flashers when slowing down unexpectedly is the most appropriate and safety-enhancing choice in the given options.