Final answer:
The simple squamous epithelium is a single layer of flat cells that functions efficiently in absorption or diffusion due to its thin, tightly knit structure, facilitating processes like passive diffusion. The correct option is b.
Step-by-step explanation:
The epithelial tissue that is a single layer of flat cells functioning in absorption or diffusion is the simple squamous epithelium. The flatness of simple squamous cells provides a short distance for the diffusion of oxygen, nutrients, and waste products. Because they are tightly knit and thin, simple squamous cells are efficient for processes such as passive diffusion, which is the movement of substances across the membrane without the input of energy, making them specialized for locations where rapid diffusion is required, such as the alveoli in the lungs or the lining of blood vessels.