Final answer:
The apical surface of epithelial tissue is the free surface (option D), which is opposite the basal surface and is specialized for functions like secretion and protection. In columnar epithelium, the cells are one layer with an apical surface adapted for absorption or secretion.
Step-by-step explanation:
The apical surface of epithelial tissue is option D) Free surface. This surface is exposed to either the external environment or an internal space within the body. Unlike the basal surface that is attached to the underlying connective tissues, or the lateral surfaces that are in contact with adjacent cells, the apical surface is on the opposite side of the basal surface and is typically specialized for specific functions such as secretion, absorption, and protection. For instance, in the lining of the small intestine, the epithelial cells have their apical surfaces adapted to release digestive enzymes and absorb nutrients. In the skin, which is part of the cutaneous membrane, the apical surface is covered with dead, keratinized cells that serve as a barrier against pathogens and prevent desiccation.
Epithelial cells exhibit polarity, with distinct differences between their apical and basal ends. Structures such as cilia may be found on the apical surface, and these are crucial in the functioning of certain epithelial tissues. When observing epithelial cells under the microscope, if the cells are arranged in one layer and appear tall and narrow, with the nucleus closer to the basal side, the tissue is columnar epithelium, not stratified epithelium which consists of multiple layers.