Final answer:
The aorta is the artery that is widened in a thoracic aneurysm, specifically the thoracic aorta.
Step-by-step explanation:
The artery that is widened in a thoracic aneurysm is the aorta. The aorta is the largest artery in the body, originating from the left ventricle and extending downwards to bifurcate at the level of the fourth lumbar vertebra into the two common iliac arteries. For a thoracic aneurysm, the relevant portion is the thoracic aorta, which gives rise to visceral and parietal branches in the thoracic region. A thoracic aneurysm implies a ballooning or widening of the aorta's wall in the chest area.