The correct answer is (b). Radiation thermometers are also known as pyrometers. They are used to measure the temperature of objects that are too hot to be touched or that are not in direct contact with the thermometer. Radiation thermometers work by measuring the amount of infrared radiation that is emitted by an object. The amount of infrared radiation emitted by an object is proportional to its temperature.
Thermo-electric thermometers (a) work by measuring the voltage that is generated when two different metals are connected together and there is a temperature difference between them. Gas thermometers (c) work by measuring the pressure of a gas. Liquid thermometers (d) work by measuring the volume of a liquid.