An athlete preparing for a marathon should focus on consuming a diet rich in:

A marathon is a long-distance endurance event covering 42.195 kilometres (26.2 miles). An athlete preparing for a marathon needs a diet that provides sustained energy over a prolonged period, which is exactly what complex carbohydrates deliver.

Complex carbohydrates (found in foods such as pasta, rice, whole grains, potatoes, and oats) are broken down gradually by the body into glucose. This slow, steady release of glucose into the bloodstream provides a continuous supply of energy throughout the race. This is why marathon runners practise carbohydrate loading (increasing carbohydrate intake in the days before a race) to maximise glycogen stores in the muscles and liver.

The other options contain incorrect pairings of nutrient and function:

• Proteins are essential for building and repairing muscle tissue, not for providing quick energy. Proteins are not the body's preferred energy source during endurance exercise.
• Vitamins are micronutrients that regulate metabolic processes and support immune function; they do not regulate body temperature.
• Fats are a concentrated energy source and provide insulation, but their primary role is not building and repairing muscle tissues - that is the role of proteins.