The answer is A, arteries.
Comparing to veins, which is the structure that does the opposite of arteries, transporting blood back to the heart, arteries have a much higher blood pressure. This also explains why artery has a thicker wall, it has to stand in such high pressure. Arteries usually carry blood with high nutrient content and high oxygen, but except for the pulmonary artery, which it transports low oxygen blood to the lungs for gas exchange.
One method to memorise that the artery transports blood away from the heart is "Artery Away" both of these letters starts with letter A which makes it easy to remember.