When signaling with sound, how long should a vessel aground signal last?

The appropriate signal for a vessel that is aground is continuous sound signaling until the vessel is freed. This means that if a vessel becomes stranded, it should continuously sound an alarm or signal to alert other vessels or nearby authorities of its situation. The rationale behind this approach is to maximize the chances of being noticed and assisted, as a continuous signal draws attention effectively.

In contrast, other options like repeating every 30 seconds or signaling at specific intervals do not adequately communicate the urgency or the critical status of being aground. These would be insufficient in conveying that immediate assistance may be needed. The other choices might imply a more routine signaling method that is not suitable for a vessel in a distress situation like being aground, making them less effective than the continuous signal specified in the correct answer.