The correct answer is option 4: Sugar.

In onions, the food is stored in the form of sugar. Sugars are simple carbohydrates that plants produce during photosynthesis. They serve as a source of energy for the plant. Onions contain different types of sugars, such as fructose and glucose. These sugars are stored in the cells of the onion bulb and provide nourishment for the plant while it grows. This stored sugar is also responsible for the sweet taste of onions when they are cooked or caramelized. The presence of sugar in the onion bulb enables it to grow and develop into a mature onion plant. Therefore, sugar is the primary form in which food is stored in onions.