Question

identify the vessel labeled a:
for clarity, blue arrows point to blue vessels; red arrows point to red vessels

Explanation:

In the neck and thoracic region, the vessel labeled A (with red arrow, so an artery) is the Common Carotid Artery. It supplies blood to the head and neck, and its position (along the neck, branching from the aortic arch - related vessels) matches. Blue vessels are veins, red are arteries; A is red - colored, so an artery, and the common carotid is a major neck artery.

Answer:

Common Carotid Artery