The correct answer is B. change food or sunlight into useful energy.
Maintaining constant internal environment by self-regulatory mechanism by an organism is called homeostasis. Like multicellular organisms, unicellular organisms such as Amoeba, Chlamydomonas also maintain constant internal environment. Each unicellular organism requires food or other energy source.to maintain water and minerals concentrations within the cell up to a certain levels and quickly to respond to changes in its environment.