The correct answer is: A. Wind
Undulating sandy plains are formed by wind erosion. Wind erosion is the process by which wind removes loose soil particles from the surface of the Earth. The wind can carry these particles long distances, and they can be deposited in new locations. When the wind blows over sand, it can pick up the sand particles and carry them away. The sand particles are then deposited in new locations, where they can form undulating sandy plains.
Glaciers are large masses of ice that move slowly over the Earth’s surface. They can erode the land by scraping and grinding away at the surface. Water is a powerful agent of erosion. It can erode the land by flowing over the surface, by carrying away loose soil particles, and by dissolving rocks. Waves are formed by the wind blowing over the surface of the water. They can erode the land by crashing against the shore and by carrying away loose soil particles.
In conclusion, the correct answer is: A. Wind.