Final answer:
The effective treatment of hypokalemia with potassium chloride infusion is indicated by the absence of U waves on the electrocardiogram, suggesting a return to normal serum potassium levels.
Step-by-step explanation:
A nurse working on a telemetry unit who initiates an infusion of potassium chloride for a client with hypokalemia needs to look for signs on the electrocardiogram (ECG) that the treatment is effective. One of the indicators of resolving hypokalemia is when U waves become less prominent or absent. Therefore, the correct answer is c. U waves are absent. This indicates that the serum potassium levels are likely returning to normal, as U waves are often associated with hypokalemia. The absence of U waves suggests that the heart's electrical activity is normalizing as the potassium levels are corrected.