B. Organism only need water.
Explanation - Living organisms need both food and water for their survival. They cannot live only on water because food is the source of energy. They get macro-nutrients like carbohydrates, proteins, fats, vitamins and minerals from the food.
Carbohydrates are the major source of energy for living organisms. It is needed for them to carry out physical work and also metabolic activities. Therefore, living only on water will make the living organism to starve and eventually die.