Final answer:
The structures of the small intestine in order from smallest to largest are microvilli, villi, folds, and then the small intestine itself, enhancing surface area for nutrient absorption.
Step-by-step explanation:
The correct order for the structures of the small intestine from smallest to largest is microvilli, villi, folds, and then the small intestine itself. The microvilli are tiny projections on the apical surface of the epithelial cells that line the villi. They vastly increase the surface area for absorption. Villi are finger-like projections from the walls of the small intestine that further increase the surface area and are covered in epithelial cells containing microvilli. The villi create folds on the inner surface of the small intestine, making the interior highly folded and allowing for an exceptional area for nutrient absorption. Therefore, the correct answer to the question is microvilli, villi, folds, small intestine.