When the Wenner method is used to test soil resistivity, the resistance measured by the test instrument is provided by the ______.

The Wenner method is a technique employed to measure soil resistivity by using four electrodes. In this setup, two inner electrodes are used to inject current into the ground, while the two outer electrodes measure the resulting voltage drop. The key aspect of the method lies in calculating the resistivity of the soil based on the voltage measured between the inner electrodes and the current flowing through the soil.

The correct choice focuses on the voltage drop between the two inner pins that are driven into the earth, as this voltage drop is directly related to the resistance encountered by the current as it flows through the soil. By knowing the current introduced and measuring the voltage drop, the tester can apply Ohm's Law (Voltage = Current x Resistance) to calculate the soil resistivity.

The other options do not accurately describe the mechanism of the Wenner method. For instance, current flow changes and grounding rod interference do not directly pertain to measuring soil resistivity in this context, nor do they provide the information needed to compute the resistivity. Therefore, emphasizing the significance of the voltage drop between the inner pins clarifies the underlying principle of the Wenner method and confirms why this is the correct choice.