Final answer:
The correct answer is D. The sun, which is the primary source of energy that drives the water cycle by causing the evaporation of water and the formation of precipitation.
Step-by-step explanation:
The major source of energy responsible for the water cycle is the sun. The sun's energy is critical as it warms the Earth's oceans and surface waters. This warmth leads to the evaporation of water and sublimation of ice, allowing large amounts of water vapor to enter the atmosphere. The water vapor then condenses into clouds and ultimately precipitates as rain or snow, which returns water to the Earth's surface. This process continually circulates water throughout the environment, sustaining the cycle. Therefore, the correct answer to the question is D. The sun.