Final answer:
The properly working pacemaker directly affects the Cardiac rhythm by sending electrical impulses to maintain a suitable heart rate and rhythm, ensuring regular beats.
Step-by-step explanation:
Which of the following is affected by a properly working pacemaker? The correct answer is C) Cardiac rhythm. A pacemaker is a device that sends electrical impulses to the heart muscle to maintain a suitable heart rate and rhythm. It directly influences cardiac rhythm, ensuring that the heart beats more regularly in those who have an abnormal heart rate or rhythm. While the pacemaker influences the timing and sequence of the heart muscle contractions, it is specifically designed to correct cardiac rhythm abnormalities like arrhythmias. Stroke volume and ejection fraction are measures of the heart's performance in pumping blood during each beat, and automaticity refers to the heart's ability to contract on its own, but both are not directly controlled by an external pacemaker.