The best reason to support the claim that this food is a good choice for the marathon runner is:
"This food has almost all sugar and would provide a quick energy source during the marathon that the runner could use easily."
This option highlights the need for quick and easily accessible energy during endurance activities like marathons, making it the most relevant justification. Marathon runners typically require carbohydrates for quick energy, and sugars can be rapidly converted into glucose for immediate use, which is crucial during prolonged physical exertion. The other options either contain misconceptions or do not directly address the runner's immediate energy needs.