The correct answer is: D. measuring a voltage.
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. If only one terminal is used, it acts as a voltage divider.
Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment and light dimmers. They are also used in many measuring instruments, such as voltmeters and ohmmeters.
Option A is incorrect because a potentiometer does not compare two voltages. It measures a voltage.
Option B is incorrect because a potentiometer does not measure a current. It measures a voltage.
Option C is incorrect because a potentiometer does not compare two currents. It measures a voltage.
Option E is incorrect because a potentiometer is a device for measuring a voltage.