Final answer:
Surface absorptive cells are the primary cells in the small intestine, containing microvilli that enhance nutrient absorption.
Step-by-step explanation:
The primary cell found in the lining of the small intestine is the surface absorptive cell, which is a type of columnar epithelial cell specialized for absorption. These cells have microvilli on their surface that significantly increase the area available for nutrient absorption. Furthermore, the villi, folds in the lining of the small intestine, also contribute to this large absorptive surface area. Both structures enhance the digestive tract's efficiency in absorbing nutrients.