The correct answer is B) Two short blasts. The U.S. Coast Guard Navigation Rules and Regulations Handbook states that the sound signal for permission to leave the riverward chamber of twin locks is two short blasts. This signal is given by the lockmaster to indicate that the vessel has permission to exit the lock. The other answer choices are incorrect because: A) One long blast is the signal to indicate that the vessel is ready to leave the lock, not permission to do so; C) One short blast is the signal to acknowledge the lockmaster's instructions; and D) Two long blasts is the signal to indicate that the vessel is not ready to leave the lock.