Final answer:
The current density during electrical stimulation is directly influenced by the size of the electrode, which changes the area over which the current is distributed, thus affecting the density of the current.
Step-by-step explanation:
The factor that influences the current density during electrical stimulation is b. Electrode Size.
Current density is defined as the current flow per unit area of cross-section (J = I/A) and is a critical factor in determining how electrical currents affect the conductive medium. The size of the electrode alters the area through which the current is distributed; a larger electrode disperses the current over a greater area, resulting in lower current density, while a smaller electrode concentrates the current, increasing the current density. This is significant for applications like electrical stimulation therapy, where appropriate current densities are crucial for safety and effectiveness. Factors like electrode placement, electrode type, and interelectrode distance also influence the effects of electrical stimulation; however, the question specifically asks about current density, which is directly related to the electrode size.