Biology question, endothelial cells

The answer is A, but I don’t understand why squamous Epithelium is an endothelial cell.

Try this article: Epithelial and Endothelial Cells- How Do They Differ? – Primary Stem Cells for Industrial & Academic Research – Kosheeka.
I think they cover it good.

It’s confusing, but endothelial cells are actually a type of simple squamous epithelial cell. They are thin, flat, and lined with blood vessels that can help exchange materials between blood and tissues. The reason they are called endothelial cells is because they are located within blood vessels, but they are actually still simple squamous epithelial cells due to their function.