Which of the following describes the main difference between energy that comes from simple foods (such as sugar or candy) and those that are complex (such as oatmeal)?

The difference is It takes time and energy for your body to break down more complex foods, whereas simple foods are broken down more quickly.
This means that if you consume simple foods such as sugar, you will receive a boost of energy for a short period of time and you will crash afterward. Meanwhile, if you consume complex carbohydrates, you will receive slow and sustainable amount of energy for a longer period of time