The correct answer is B) Only when there is a chance of their being seen by rescue vessels. The reasoning behind this is that distress flares and rockets should only be used when there is a reasonable chance that they will be observed by rescue vessels or aircraft. These signals are meant to alert potential rescuers to the vessel's distress, so activating them when no help is likely to be nearby would be a waste of resources. The other options, such as using them at set time intervals regardless of the situation, are not appropriate as that could lead to the signals being ignored or wasted. Distress flares and rockets should be used judiciously, only when they can effectively communicate the need for assistance.