Key Concept: Dietary fiber is unique among nutrients because the human body cannot digest it.
Why it's correct: Since fiber isn't broken down, it adds bulk to stool and helps regulate bowel movements, directly aiding digestion and preventing constipation.
Why others fail: Providing energy and building muscle both require the body to digest and absorb a nutrient, which doesn't happen with fiber; 'enhancing taste' isn't a real nutritional function.