The descriptive term for plants that are sturcturally adapted to withstand protracted dry conditions is .

Xerophytic

Explanation:

It is xerophytic and it is gotten from Greek word, Xeros( dry) and phutons(plants). It is an adaptive term that is use to describe plants species that posses structural adaptive features which make them to adapt and survive in areas where there is little water or dry region like desert, or area covered with ices by possessing some mechanism which help them to reduce water loss or concerns water. Examples include cactus, pineapple and some Gymnosperm plants. Some of these plants species posses extensive root system and their leaves are reduced to cacti which does not support water loss.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.