Final answer:
The 'brush border' of the small intestine refers to the microvilli, which are cylindrical extensions on epithelial cells that significantly enhance the absorptive surface area.
Step-by-step explanation:
The structure of the small intestine is specially adapted for the absorption of nutrients. The correct match for the structure known as the brush border is b) Microvilli. These are tiny, cylindrical extensions of the plasma membrane found on the apical surface of the epithelial cells in the mucosa of the small intestine.
The microvilli drastically increase the surface area of the plasma membrane, enhancing its absorptive capability. Moreover, the brush border of microvilli holds enzymes that are crucial for the final stages of digesting carbohydrates and proteins. Each square millimeter of the small intestine contains approximately 200 million microvilli, highlighting the extent to which they increase the surface area for absorption.