In which situation is it least important to check your mirrors?

Answer: B — When descending a large hill

Checking your mirrors is essential for maintaining awareness of your surroundings and ensuring safety while riding. When descending a large hill, the primary focus tends to be on controlling your speed and maintaining stability, rather than monitoring nearby vehicles. This scenario could lead to fewer potential hazards from other vehicles, especially if they are not in close proximity. Conversely, making a left turn, changing lanes, and entering a roundabout involve interactions with other vehicles and require awareness of surrounding traffic to avoid collisions. In these situations, checking mirrors is crucial for determining the positions and speeds of other road users, allowing for safe maneuvering. Thus, while mirror checks are vital in most riding situations, they become less critical when dealing with straightforward downhill riding where other vehicles pose minimal risk.