Which of the following substances begins digestion in the small intestine?

Fats are the substances that begin digestion in the small intestine. When food enters the small intestine, it is mixed with bile, produced by the liver and stored in the gallbladder, which emulsifies fats. This process breaks down large fat globules into smaller droplets, making it easier for digestive enzymes, particularly lipases, to further digest these fats into fatty acids and glycerol.

While fiber, proteins, and carbohydrates are important components of digestion, their primary degradation begins in different locations of the digestive tract. Proteins are primarily broken down in the stomach by the action of gastric acids and enzymes such as pepsin. Carbohydrates begin digestion in the mouth with enzymes like salivary amylase and continue in the small intestine but do not start there. Fiber is not digested in the traditional sense; it passes unchanged through the digestive system, aiding in digestion and preventing constipation.

Therefore, the unique role of the small intestine in the digestion of fats directly relates to the emulsification process that occurs here, making fats the correct answer.