Final answer:
The sinoatrial (SA) node typically depolarizes at a rate of 80-100 beats per minute under normal conditions, which is not listed among the options. The closest correct answer based on the average resting heart rate for most individuals is 75 times per minute.
Step-by-step explanation:
The sinoatrial (SA) node, known as the heart’s natural pacemaker, depolarizes at an inherent rate, which is the fastest among the heart's conductive cells. In the absence of nervous or endocrine influence, the SA node would typically initiate a heart impulse 80-100 times per minute. However, this rate is not one of the provided options. We must then consider the average resting rates for most individuals, and specialized situations, like exceptionally trained aerobic athletes, who can have rates as low as 30-40 beats per minute.
A rate below 50 beats per minute is considered bradycardia, which is harmful. None of the listed rates matches the inherent rate of the SA node, but option d. 75 times per minute is closest to the normal physiological resting rate for most individuals, which is generally slightly slower than the node's highest inherent rate.