The best answer is C. native vegetation. This is because:
*Native vegetation is adapted to the local climate and soil conditions, and therefore requires less water than non-native plants1.
*Native vegetation also supports local biodiversity, reduces erosion, and enhances the aesthetic value of the landscape2.
*Potable water is not a good option for water-efficient landscape design, as it is a scarce and valuable resource that should be conserved for human consumption and hygiene3.
*Sprinkler systems are not a good option for water-efficient landscape design, as they can waste a lot of water through evaporation, runoff, and overspray4. Drip irrigation or micro-sprinklers are more efficient methods of watering plants.
*Permeable pavement is not a good option for water-efficient landscape design, as it is not a plant-based element, but a hardscape feature that allows water to infiltrate into the ground. Permeable pavement can reduce stormwater runoff and pollution, but it does not directly contribute to water conservation in the landscape.
References: 1: Water Efficient Landscaping Ideas: 5 Water Wise Tips 2: [Native Plants for Water Conservation] 3: [Water Conservation | US EPA] 4: [Sprinkler Irrigation - an overview | ScienceDirect Topics] : [Drip Irrigation - an overview | ScienceDirect Topics] : [Permeable Pavement - an overview | ScienceDirect Topics] : [Permeable Pavement Benefits and Costs]