Which of the following is considered a systemic cardiac vein?

The oblique vein of the left atrium is categorized as a systemic cardiac vein because it serves to collect blood from the left atrium and drain it into the coronary sinus, which is a major vein that empties into the right atrium. This function is significant as it is involved in the venous return system of the heart.

In contrast, the left superior pulmonary vein and the right inferior pulmonary vein are not considered systemic veins; instead, they transport oxygenated blood from the lungs to the left atrium. The aorta, while a major vessel that transports oxygenated blood away from the heart to the systemic circulation, is an artery rather than a vein. Therefore, the oblique vein's role in the venous drainage from the heart makes it the suitable answer in this context.