To support the claim that this food is NOT a good choice for a marathon runner, the best evidence would be:
This food is mostly carbohydrate and not enough protein.
Marathon runners primarily need carbohydrates for energy, so if the food is low in carbohydrates, it would not be suitable for their energy needs during long endurance activities.
However, as it seems like descriptions were modified slightly regarding the content types, you would want to choose an option that clearly indicates an imbalance in macronutrients favoring fat, sodium, or protein over carbohydrates:
Among the given options, the strongest evidence would be:
This food is mostly fat and not enough carbohydrate.
This is because fats are not the primary fuel source for high-intensity endurance activities such as running a marathon; therefore, a food high in fat and low in carbohydrates would not be a good choice for a runner.