Which of the following cannot be used as standard solution for determination of cell constant of conductivity cell?

Saturated KCl Solution :
A saturated solution of KCl is not typically used as a standard solution for conductivity calibration. The reason is that a saturated KCl solution may have variable composition due to temperature changes, partial crystallization, and difficulty in ensuring a uniform, reproducible concentration. Thus, it does not have a precisely known or easily reproducible conductivity value as the other standard concentrations do.
Conclusion :
Out of the given options, the saturated KCl solution (Option D) is not suitable for use as a standard solution in determining the cell constant because its concentration and thus its conductivity are not stable or easily reproducible.