The correct answer is: A. Another emf.
A self-balancing potentiometer is a device that uses a potentiometer to measure the voltage of another voltage source. The potentiometer is a three-terminal resistor with a sliding contact that can be moved along the resistor’s length. The voltage across the resistor is proportional to the position of the contact.
In an X-Y recorder, the self-balancing potentiometer is used to plot the voltage of one signal as a function of the voltage of another signal. The two signals are applied to the potentiometer’s terminals, and the contact is moved until the voltage across the resistor is equal to the voltage of the second signal. The position of the contact is then recorded on the X-Y recorder’s paper.
The other options are incorrect because they are not voltages. Frequency is a measure of how often a signal repeats itself, time is a measure of how long something takes, and pressure is a measure of how much force is applied to an area.