Final answer:
Capillaries are the blood vessels with thin, leaky walls necessary for the exchange of substances between the blood and tissues.
Step-by-step explanation:
The blood vessels that have thin, leaky walls are C) capillaries. Capillaries are the smallest blood vessels and they connect arterioles with venules, forming capillary beds where the exchange of substances between the blood and surrounding tissues takes place. The walls of capillaries are composed of a single layer of epithelial cells, which makes them well-suited for the exchange because substances such as water, oxygen, glucose, and waste products like carbon dioxide can pass through easily due to the thinness of the capillary walls. For capillaries to fulfill their function, they must be leaky, allowing substance passage, and they come in various types with different levels of 'leakiness', including continuous, fenestrated, and sinusoid capillaries.