Final answer:
Gap junctions are the cell junctions not involved in providing mechanical strength to epithelial sheets, instead, they enable cellular communication.
Step-by-step explanation:
The epithelial cell junction not involved in providing mechanical strength to an epithelial sheet is C. Gap junctions. While desmosomes, tight junctions, and adherens junctions all contribute to the physical strength and stability of epithelial cell layers, gap junctions do something quite different. They form intercellular passageways that allow for the transfer of ions and small molecules between cells, facilitating rapid communication and thus ensuring coordinated function across cell layers.