How can a diver safely extend their time-to-reserve during a dive?

To ensure a safe extension of time-to-reserve during a dive, decreasing the depth of the dive is the most effective method. At greater depths, the pressure increases, and divers consume air at a faster rate due to the effects of increased pressure on gas uptake. By reducing depth, the diver not only consumes air more slowly but also benefits from a more efficient air consumption rate due to the reduced environmental pressure.

Thus, remaining at shallower depths allows for a longer duration of air supply while maintaining safety, as it diminishes the potential risk of nitrogen narcosis and other pressure-related complications.

The other options do not promote safety in extending time-to-reserve. Increasing swimming speed leads to higher air consumption, while using less air could compromise safety if it pushes the diver too close to reserve levels. Lastly, staying at the surface longer is not a feasible option during a dive and does not directly contribute to extending time-to-reserve at depths.