During heavy exercise, the body needs more oxygen to supply the muscles and remove carbon dioxide. This increased demand for oxygen is met by an increase in breathing rate. Option 1, 15 breaths per minute, seems too low considering the average adult breathing rate at rest is around 12-20 breaths per minute. Option 2, 20 breaths per minute, is within the normal range for breathing rate during exercise but may not account for the maximum rate during heavy exercise. Option 3, 25 breaths per minute, is a plausible choice as it reflects an elevated breathing rate during intense physical activity. Option 4, 30 breaths per minute, is a higher rate but may be too extreme for an average adult during heavy exercise. Therefore, option 3, 25 breaths per minute, is the most appropriate answer provided.