Final answer:
Fibronectin is the protein fiber that promotes cell adhesion and holds cells in position.
Step-by-step explanation:
The type of protein fiber that promotes cell adhesion and holds cells in position is fibronectin. Fibronectin is an adhesive protein that is part of the extracellular matrix (ECM) and plays a crucial role in cell attachment. It interacts with integrin receptors on the cell surface, helping cells adhere to the ECM and maintain their position.