The correct answer is C. D.C. potentiometer.
A D.C. potentiometer is a voltage measuring device that can be used to measure the true open circuit e.m.f. of a battery. It does this by creating a voltage divider circuit with a known resistance. The voltage across the known resistance is then compared to the voltage across the battery, and the ratio of these two voltages is used to calculate the battery’s e.m.f.
An ammeter is a device that measures current. It does this by passing the current through a coil of wire, which creates a magnetic field. The strength of the magnetic field is proportional to the current, and this can be measured using a magnetometer. However, an ammeter cannot be used to measure the true open circuit e.m.f. of a battery because it will draw current from the battery, which will affect the battery’s voltage.
A known resistance is a resistor with a known value. It can be used to create a voltage divider circuit with a D.C. potentiometer. The voltage across the known resistance is then compared to the voltage across the battery, and the ratio of these two voltages is used to calculate the battery’s e.m.f.
None of the above is the correct answer because it does not specify a particular device.