Question

what type of tissue is enclosing these red blood cells?

Explanation:

Red blood cells are in blood vessels, and the tissue enclosing them (lining blood vessels) is simple squamous epithelium. Simple squamous epithelium consists of a single layer of flattened cells, which matches the description "single layer of flattened cells" in the image. This tissue is part of epithelial tissue, and its structure (single, flattened cells) is characteristic of simple squamous epithelium, which lines blood vessels (endothelium) to allow for efficient diffusion and blood flow.

Answer:

Simple squamous epithelial tissue (specifically endothelial tissue, a type of simple squamous epithelium, that lines blood vessels and encloses red blood cells within the vascular system)