The temperature of the sun is measured with
(a). Platinum thermometer
(b). Gas thermometer
(c). Pyrometer
(d). Vapour pressure thermometer

- Hint: In order to solve this question first we will define each thermometer which is given in option and then according to the property of the thermometers we will proceed further to find which one is suitable for measuring the temperature of sun.

Complete step-by-step solution -
Platinum thermometer: A thermometer with platinum resistance (PRT) is a device that determines temperature by measuring the electrical resistance of a piece of pure platinum wire.
Gas thermometer: A gas thermometer measures the temperature of a gas by volume difference or pressure.
Pyrometer: A pyrometer is a non-contact device detecting and measuring thermal radiation, a method known as pyrometry. This instrument can be used to calculate the surface temperature of an object.
Vapor pressure thermometer: a thermometer used to measure the temperature by the variable saturated vapor pressure of a volatile liquid
As the sun's temperature is very high, a temperature calculation of the non-contact form is needed. Therefore pyrometer is used to measure the sun's temperature.
Hence, the temperature of the sun is measured with pyrometer.
So, option (C) is the correct answer.

Note- The platinum resistance thermometer is a flexible temperature measurement device within the range from. It is used for highest accuracy precision measurements, as well as for routine industrial work. A gas thermometer, also called a gas expansion or gas actuation thermometer, can measure a wide variety of temperatures. A vapor pressure thermometer is a thermometer used to measure the vapor pressure of a liquid using a pressure gage.