The conjunctiva is referred to as a membrane because it is a thin, transparent layer of tissue that covers the inner surface of the eyelids and extends over the white part of the eyeball (sclera). This membrane serves several important functions, including protecting the eye and keeping it moist by producing mucus and tears. The conjunctiva also plays a role in the immune response, helping to defend against pathogens that might enter the eye.
In contrast, the sclera is the tough, outer layer of the eye that provides structure and protection—not classified as a membrane. The cornea, while also a crucial part of the eye responsible for focusing light, is a clear layer and does not function as a membrane in the traditional sense. The choroid coat is a vascular layer containing connective tissue that provides nutrients and oxygen to the outer layers of the retina, but it is considered more of a layer than a membrane. Therefore, the conjunctiva is the correct answer, as it fits the definition of a membrane in anatomical terminology.