Which of the following is a method of exterior design using plants that conserve water?

Xeriscaping is a landscaping approach specifically designed to reduce or eliminate the need for supplemental irrigation, making it an effective method for conserving water. This technique involves the selection of drought-resistant plants and the use of appropriate landscaping practices that minimize water usage.

In xeriscaping, elements such as mulching, soil amendment, and proper plant placement are utilized to create an efficient design that naturally retains moisture and supports the sustainable use of water resources. It is particularly relevant in arid and semi-arid regions, like Texas, where water conservation is crucial due to limited rainfall.

The other methods mentioned do not primarily focus on conserving water through exterior design. Hydroponics involves growing plants in a water-based, nutrient-rich solution, which does not apply to outdoor landscape design. Traditional landscaping often utilizes a wide variety of plants without necessarily focusing on water efficiency, potentially leading to higher water use. Organic gardening prioritizes the use of organic methods for growing food, but does not inherently address water conservation in the way that xeriscaping does. Therefore, xeriscaping is the most suitable method described in the question for water conservation in plant design.