Capillaries are best described as:

Capillaries are best described as microscopic blood vessels that play a crucial role in the circulatory system. Their main function is to facilitate the exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and the body's tissues. Being only one cell thick, capillaries allow for efficient diffusion of these substances, making them essential for maintaining tissue health and supporting metabolic processes. This unique structure and function distinguish capillaries from other blood vessels, allowing them to connect arterioles and venules and thus serving as the sites of exchange in the microcirculation.

The other choices do not accurately reflect the nature of capillaries. Major arteries are large blood vessels that carry blood away from the heart, while large vessels that transport blood refer more broadly to arteries and veins. Small lymphatic vessels are part of the lymphatic system, which is separate from the circulatory system.