Final answer:
An increased heart rate of 123 bpm in a client with atrial fibrillation leads to decreased ventricular filling time, affecting cardiac output negatively.
Step-by-step explanation:
The nurse cares for a client with atrial fibrillation and a rate of 123 bpm. The heart rate affects cardiac output for this client by decreasing ventricular filling time, because as the heart rate increases, there is less time spent in diastole, which means there is less time for the ventricles to fill with blood. The correct answer is A. Ventricular filling time is decreased.