Which of the following is a sign that a diver should slow their ascent?

Experiencing visual changes is a significant indication that a diver should slow their ascent. Changes in vision can signal the onset of decompression sickness or other altitude-related issues, such as nitrogen narcosis or visual disturbances associated with ascending too quickly. It is crucial for divers to pay close attention to their physiological responses during the ascent, as visual changes can indicate that they are reaching a level of stress that may require them to descend slightly or slow down their ascent rate to prevent serious injury.

In contrast, feeling euphoric could reflect nitrogen narcosis, but it is not a direct sign to slow the ascent—it's more of an indication of being affected by ascending or diving conditions. Increased buoyancy may require adjustments while ascending but does not directly signal the need to slow down. Feeling slightly cold is not a primary concern concerning ascent speed; rather, it can simply indicate that a diver is starting to feel the effects of temperature changes in the water. Overall, the presence of visual changes is a clear and urgent sign that should prompt a diver to reassess their ascent strategy.