Final answer:
The Purkinje fibers in the heart conduction system have the slowest rate of firing.
Step-by-step explanation:
d. The slowest rate of firing in the heart conduction system would be in the Purkinje fibers. These specialized fibers conduct the electrical impulse from the bundle branches to the ventricular myocardium, causing the ventricles to contract. This delay allows the atria to empty their contents into the ventricles before the ventricles pump out the blood.