Final answer:
The mucus produced by intestinal epithelial cells functions to protect against microbial invasion by forming a physical and antimicrobial barrier.
Step-by-step explanation:
The thick layer of mucus produced by intestinal epithelial cells primarily functions to protect against microbial invasion. Goblet cells, which are a type of simple columnar epithelial cell, secrete a gel-forming mucin that is the major component of this protective mucus. This mucus layer not only acts as a physical barrier but also contains antimicrobial agents that help to reduce the risk of pathogens reaching deeper tissues. Additionally, the production of mucus is characteristic of the gut epithelium, helping maintain the mucous membrane that lines the inner surfaces of the body.
The answer to the student's question is b) Protect against microbial invasion.