Which of the following factors increases the risk of hyperthermia during diving?

Prolonged exposure to the sun significantly increases the risk of hyperthermia during diving because it can lead to an increase in body temperature. When divers spend extended periods in the sun before or between dives, their bodies can become overheated, particularly if they are not properly hydrated or if they are wearing thick wetsuits or other insulating gear that traps heat. This elevated body temperature, coupled with the physical exertion associated with diving, can result in a higher risk for heat-related illnesses such as hyperthermia.

In contrast, low-altitude diving does not inherently contribute to a risk of hyperthermia, as it typically does not affect body temperature regulation significantly. Cold water temperatures tend to decrease the body's temperature, thus posing a risk of hypothermia rather than hyperthermia. Short surface intervals can also play a role in decompression sickness but are less directly related to the regulation of body temperature. Thus, prolonged exposure to the sun stands out as the correct factor related to increased hyperthermia risk while diving.