Final answer:
To increase his fiber intake, John can replace the cornflakes with whole-grain cereal or oatmeal and add fruits such as berries or sliced banana to his breakfast.
Step-by-step explanation:
Diverticular disease is a condition where pouches in the wall of the large intestine become infected and inflamed. To increase fiber intake, John can make some modifications to his usual breakfast. He can replace the cornflakes with whole-grain cereal or oatmeal, which are high in fiber. He can also add fruits, such as berries or sliced banana, to his cereal or yogurt.