Which word best describes a "membrane"?

The term "membrane" is most accurately described as a thin, pliable sheet of material. In biological contexts, membranes are often composed of lipid bilayers that create semi-permeable barriers, allowing for the selective exchange of substances in and out of cells. This thinness and flexibility are essential characteristics, as they enable membranes to function properly within the dynamic environments of living organisms.

In contrast, the other options describe structures that do not align with the properties of a membrane. A solid, rigid structure may imply a lack of flexibility, which is contrary to the dynamic nature of membranes. A dense and thick layer suggests additional mass and rigidity, which does not fit the typically thin and adaptable quality of membranes. Finally, a thick block of substance describes an entirely different form of material that lacks the defining characteristics of membranes, which are characterized by their thinness and flexibility allowing for various biological functions.