Step 1: Understanding the Concept
The small intestine mucosa has finger-like villi. The mucosa also dips into pits between their bases.
Step 2: Name the pits
These pits hold the intestinal glands that secrete succus entericus and are called crypts of Lieberkuhn.
Step 3: Why the other options fail
The sphincter of Oddi guards the opening of the hepatopancreatic duct. The Sylvian sulcus is a brain groove, and the pulp cavity lies inside a tooth.
Final Answer:
The cavities are the crypts of Lieberkuhn.
\[ \boxed{\text{(C) }\text{crypts of Lieberkuhn}} \]