The correct answer is B) when all other means have failed. A tourniquet should only be used as a last resort to control life-threatening bleeding when all other means, such as direct pressure and elevation, have failed. Tourniquets can cause significant tissue damage if left on for too long, so they are not appropriate for minor wounds. Unconsciousness is also not a criterion for using a tourniquet, as the focus should be on the severity of the bleeding. Tourniquets are generally not used for puncture wounds, as they are more effective for controlling bleeding from major limb injuries.