The process of maintaining a stable internal environment regardless of external conditions is known as homeostasis. Organisms that carry out homeostasis do so in order to maintain a constant, optimum environment for the cells within their body. This optimum environment includes maintainance of temperature, pH of body fluid, and control of concentration of substances that are dissoled in body fluids, such as glucose and urea.
The correct answer would be letter B: water molecules split into oxygen and hydrogen atoms. This is how plants convert water into a usable source of food and energy to support its photosynthetic capacity to power the plant within its environment.