The correct answer is option C. Thermocouples are made by two dissimilar metals. These metals are joined at both ends to form two junctions. When there is a temperature gradient between the junctions, a voltage is generated due to the Seebeck effect. This voltage is proportional to the temperature difference and can be measured to determine temperature accurately. Therefore, option C, two dissimilar metals, accurately describes the composition of thermocouples, which utilize the thermoelectric effect to measure temperature differences. Options A and B are incorrect as thermocouples require dissimilar metals to function effectively, and option D is false as thermocouples are indeed made from two dissimilar metals.