Decompression sickness symptoms usually appear:

Decompression sickness, also known as "the bends," is a condition that arises when a diver ascends too quickly after being submerged at depths where nitrogen is absorbed into the body under pressure. The symptoms of decompression sickness may not appear immediately after surfacing due to the gradual release of nitrogen from the tissues.

The correct choice indicates that symptoms typically manifest between 15 minutes to 12 hours after surfacing. This timeframe accounts for the fact that nitrogen can take time to come out of solution in a diver's body as pressure decreases upon ascent. Factors such as the depth of the dive, the duration of the dive, and the diver's individual physiology can all affect when symptoms present, which is why this range is considered accurate.

Immediate symptoms would suggest a problem like an air embolism rather than decompression sickness. Symptoms within an hour could fit within the broader context of decompression sickness but do not accurately encompass the more extended period that can occur. Lastly, while experiencing multiple dives can increase the risk of decompression sickness, it is not a prerequisite for the condition to manifest within the specified timeframe outlined in the correct answer.