Through which vessels does blood enter the Left Atrium?
Oxygenated blood enters the Left Atrium via the Pulmonary Veins.
Recall that normally, veins carry deoxygenated blood. The pulmonary veins are the exception to the rule.
However, if you think of the rule more of as veins carry blood toward the heart and arteries carry blood away from the heart, then you won't go wrong.