The correct answer is: B. Make densely populated areas easy to walk.
Smart growth refers to solutions that support economic growth and contribute to development of healthy environment and strong communities. It includes strategies that preserve natural lands and critical environmental areas, and also protect water and air quality. Important components of smart growth are reusing already-developed land, making walkable neighborhood, preservation of open space etc. One of the components is the use of mixed lands, which means that residential, commercial and recreational objects should be in close to one another.