Final answer:
Capillaries have the highest cross-sectional area due to their extensive network of capillary beds, despite their individual small diameter.
Step-by-step explanation:
The blood vessels that have the highest cross-sectional area are capillaries. While the diameter of an individual capillary is much smaller than that of other types of vessels such as arterioles, the number of capillaries in the body is substantially greater, resulting in a larger total cross-sectional area.
Capillaries form extensive networks called capillary beds, which facilitate the exchange of substances between the blood and the tissues. This vast network provides a cross-sectional area far greater than that of veins, arterioles, or venules.