The correct answer is option 3 - cellulose. A fiber-rich diet includes foods that are high in cellulose. Cellulose is a type of complex carbohydrate that is found in the cell walls of plants. It is a form of dietary fiber that is not easily broken down by the human digestive system.

Glycogen (option 1) is a form of stored glucose in the liver and muscles and is not considered a fiber.

Proteins (option 2) are necessary for growth and repair of tissues, but they are not considered dietary fiber.

Fats (option 4) are a type of macronutrient that provides energy, but they are also not considered dietary fiber.

Fiber is important for maintaining a healthy digestive system, promoting bowel regularity, and preventing constipation. It also helps to control blood sugar levels, lower cholesterol levels, and promote a feeling of fullness, which can aid in weight management. Good sources of dietary fiber include whole grains, fruits, vegetables, legumes, and nuts.