As the fill factor decreases, the impedance variation of a pickup coil produced by a change in conductivity will:

When the fill factor of a pickup coil decreases, it means that there is less of the coil's area occupied by the conductive material being tested. The fill factor is essentially the ratio of the area of the coil that is filled by the target material to the total area of the coil. A lower fill factor implies that a smaller amount of conductive material is interacting with the coil.

As conductivity changes in a material, the overall impedance of the coil will vary based on how much of the coil is interacting with the conductive material. With a decrease in fill factor, the coil's response to changes in conductivity diminishes because there is less conductive material affecting the magnetic field and the generated eddy currents. This results in a decrease in the impedance variation that the coil can detect as conductivity changes.

Therefore, as the fill factor decreases, the impedance variation of the pickup coil produced by a change in conductivity will decrease, confirming that the correct answer is that it will decrease. Understanding the relationship between fill factor, conductivity, and impedance is crucial in evaluating the effectiveness of eddy current testing.