The correct answer is C) May be used separately or with other distress signals. A distress signal is used to indicate that a vessel or person is in grave and imminent danger and requires immediate assistance. As per the regulations, a distress signal can consist of various methods, such as visual signals (e.g., flares, flags), audible signals (e.g., prolonged blasts on a fog signal), or radio signals. The key point is that a distress signal can be used on its own or in combination with other distress signals to maximize the chances of being seen or heard by nearby vessels or rescue authorities. The other answer choices do not accurately describe the nature and usage of a distress signal.