Final answer:
The body primarily uses fat as its fuel source at rest, with fat option (d) being a more efficient energy source for ongoing needs, unlike carbohydrates, proteins, vitamins, minerals, or water.
Step-by-step explanation:
At rest, the body's fuel source is primarily fat option (d). While carbohydrates, proteins, and fats are all primary components of food and serve as energy sources, when the body is at rest, it tends to use more fat for energy because it's a more efficient fuel source for slow, ongoing needs. The body stores excess energy intake as glycogen in the liver and muscle cells, and in fat cells as adipose tissue. Vitamins and minerals, while essential, are not primary fuel sources. They are involved in various cellular functions and metabolic pathways but do not directly provide energy. Water is crucial for life but also does not serve as a fuel source. Therefore, the correct answer is d. fat.