Which substance is utilized by all body cells as fuel?

Glucose is the preferred source of energy for all body cells, serving as a primary fuel for cellular metabolism. It is a simple sugar and a key carbohydrate that can be easily utilized by the body's cells for immediate energy needs. Glucose is broken down through a series of biochemical processes to produce ATP (adenosine triphosphate), which is necessary for various cellular activities.

While other substances like fatty acids and amino acids can also be used for energy, they often require more complex processes or are used under specific circumstances. For instance, fatty acids are primarily utilized by muscle cells and during extended periods of low-intensity exercise. Amino acids are generally used for protein synthesis or when carbohydrate availability is low.

Glycogen, on the other hand, serves as a stored form of glucose in the liver and muscles and can be converted back to glucose when needed. However, it is not directly utilized as fuel itself; it must first be broken down into glucose.

Thus, glucose stands out as the fuel source universally used by all cells for energy, making it the correct answer.