Which of the following substances cannot be broken down by the digestive system?

Fiber is the correct choice because it is a type of carbohydrate that the human digestive system cannot fully break down. While enzymes in the digestive tract can break down starches and sugars into simpler sugars and proteins into amino acids, fiber resists these enzymatic actions.

Fiber includes both soluble and insoluble forms, which serve different functions in the body, such as promoting bowel regularity and aiding in cholesterol management. Insoluble fiber, in particular, passes through the digestive system mostly intact, contributing bulk to stool while facilitating the movement of waste.

In contrast, starch, sugars, and proteins are all broken down during digestion into their respective simpler components—glucose and other monosaccharides for starches and sugars, and amino acids for proteins—allowing the body to absorb and utilize them for energy and cellular functions.