The correct answer is: A voltage of about 200 V can be measured by a D.C. potentiometer in conjunction with a volt ratio box.
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If the voltage across the resistor is known, the voltage across the sliding contact can be calculated. A volt ratio box is a device that allows the user to select different voltage ratios. By connecting a potentiometer to a volt ratio box, the user can measure voltages that are higher than the maximum voltage that the potentiometer can measure directly.
Option B is incorrect because a D.C. potentiometer cannot measure voltages that are higher than its maximum voltage rating. Option C is incorrect because a D.C. potentiometer cannot measure voltages that are higher than its maximum voltage rating without the use of a volt ratio box. Option D is incorrect because a voltage of about 200 V can be measured by a D.C. potentiometer in conjunction with a volt ratio box.