What structure consists of a sheet of simple squamous epithelial cells that folds onto itself to form a double layer?

The correct answer is the serous membrane, which is indeed comprised of a sheet of simple squamous epithelial cells that folds onto itself, creating a double layer. This structure lines body cavities and covers the organs within those cavities. The two layers of the serous membrane are called the parietal layer, which lines the walls of the cavity, and the visceral layer, which covers the organs themselves. This arrangement allows for the creation of a lubricating fluid, serous fluid, between the two layers, reducing friction as organs move within the cavities.

Other membranes mentioned in the options serve different functions and have distinct structures. The synovial membrane, for example, lines the cavities of joints and produces synovial fluid but is composed of a different type of connective tissue, not simple squamous epithelium. The basement membrane is not a membrane in the same sense as the serous or mucous membranes, but instead it acts as a thin layer of connective tissue that supports epithelial cells and anchors them to underlying structures. The mucous membrane lines body cavities that are open to the exterior and typically contains goblet cells that produce mucus, but it does not form the double-layer structure characteristic of serous membranes.