What is considered a mandatory distress signal when at anchor?

When a vessel is at anchor and wishes to signal a distress situation, no specific mandatory distress signal exists under the COLREGs (International Regulations for Preventing Collisions at Sea) for that situation.

While flares, sound signals, and radio signals are all recognized means to signal distress, their use at anchor is not mandated in the same way it might be when under way. For instance, flares are typically used to signal distress when a vessel is in motion or in a situation that requires immediate attention over a wide area. Sound signals can convey various messages, but in the context of a vessel at anchor, they do not constitute a mandatory distress signal. Radio signals can be used to communicate distress, but again, there is no specific requirement for an anchor situation.

In summary, the situation of being at anchor does not present specific mandatory distress signals according to the COLREGs, making the answer indicating neither flares, sound signals nor radio signals being mandatory in that context the best choice.