Explanation:

- Option 1: Fats

- Fats are a significant energy source.

- They assist in nutrient absorption but are digestible.

- Option 2: Roughage ()

- Roughage, also known as dietary fiber, is the indigestible part of the food.

- It is crucial for maintaining digestive health and regular bowel movements.

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- Option 3: Carbohydrates

- Carbohydrates are the primary source of energy and are digestible.

- They break down into sugars.

- Option 4: Proteins

- Proteins are vital for building body tissues.

- They are digestible and broken down into amino acids.

Roughage is the correct answer as it plays a key role in digestion but is not absorbed as other nutrients are.