The correct answer is: A. a D.C. potentiometer directly.
A potentiometer is a voltage divider used for measuring electric potential (voltage); the component is an implementation of the potentiometer principle. A potentiometer is essentially a voltage divider used to compare an unknown voltage to a reference voltage. A potentiometer consists of three terminals: a voltage input terminal, a voltage output terminal, and a reference terminal. The output voltage is proportional to the ratio of the resistances between the input and output terminals.
To measure a direct current (DC) voltage, a potentiometer can be used directly. The potentiometer is connected in series with the DC voltage source and a known resistance. The output voltage of the potentiometer is adjusted until it is equal to the known voltage. The current through the potentiometer is then equal to the DC voltage divided by the known resistance.
Option B is incorrect because a D.C. potentiometer in conjunction with a standard resistance is used to measure an unknown resistance.
Option C is incorrect because a D.C. potentiometer in conjunction with a volt ratio box is used to measure a high voltage.
Option D is incorrect because a D.C. potentiometer can be used directly to measure a DC voltage.