Final answer:
Depolarization occurs first in the endocardium as it is closer to the heart's conduction system, including the SA and AV nodes, ensuring the atria contract before the ventricles.
Step-by-step explanation:
Depolarization in the heart occurs first in the endocardium before the epicardium. This is because the endocardium, the inner layer of the heart wall, is closer to the conduction system of the heart, specifically to the sinoatrial (SA) node and the atrioventricular (AV) node. The electrical signals that initiate the heartbeat and trigger contraction begin at the SA node, then travel to the AV node before spreading through the walls of the ventricles. This conduction pathway ensures that the atria contract first, pushing blood into the ventricles before the ventricular contraction happens. Therefore, the correct answer to the question is B) Endocardium; Closer proximity to the sinoatrial (SA) node.