During an aerobic workout, you depend on liver glycogen to keep your blood sugar levels steady. With full liver glycogen stores, a less fit athlete will run out of glycogen after one hour and 45 minutes of exercise. Once liver glycogen is gone, your blood sugar levels drop and your workout or event is negatively affected.
A. would be the correct answer
I believe the answer is: A. more than 90 minutes
This happen because muscles would typically depleted of oxygen after 90 minutes of continuous exercise. Because of this, carbo loading for less than 90 minutes exercise would only add unnecessary calories that might prevent you from reaching your goal.