Final answer:
The unit of bone that consists of a central canal surrounded by concentric rings of calcified matrix is called an osteon. So, the correct answer is option a.
Step-by-step explanation:
The unit of bone that consists of a central canal (containing blood vessels, nerves, and lymph vessels) surrounded by concentric rings of calcified matrix is called an osteon. Each osteon is composed of lamellae, which are concentric rings of calcified matrix. The central canal, also known as the Haversian canal, contains the blood vessels, nerves, and lymphatic vessels.
So, the correct answer is option a.