The blood vessels that have the smallest diameter and consist of a single layer of simple squamous epithelial cells so they may facilitate the exchange of substances are

Capillaries

Explanation:

Capillaries are one of the three major blood vessels in humans (the other two being; veins and arteries). They are known to connect the arteries and the veins together in the circulatory system. Capillaries majorly function in the exchange of substances e.g oxygen, nutrients between the blood and tissues.

Due to their role in exchanging substances via diffusion, capillaries contain the simple squamous type of epithelial tissue, which helps them perform this function. Furthermore, capillaries are the smallest in diameter and most abundant in number of all blood vessels.

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