A signal whistle's audible sound may be heard up to _____________ yards.

The correct answer indicates that a signal whistle's audible sound may be heard up to 400 yards. This distance is based on the design and intent of the signal whistle, which is specifically made to be a safety device for communication in maritime environments. Its piercing sound is engineered to carry over water and through various environmental conditions, making it an effective tool for signaling distress, ensuring that the sound can alert nearby vessels or personnel.

Understanding the effective range of the whistle is critical for boat crew members, as it impacts the ability to communicate in emergencies. A range of 400 yards allows crew members to signal to others, thereby enhancing safety during operations. This range is also a standard reference used in various training materials to ensure crew members are prepared for real-world situations where signaling may be necessary for rescue or safety communication.