Concept:A standard thermometer is required as a reference to calibrate other thermometers accurately.
Explanation:The constant volume gas thermometer measures temperature by observing the pressure change of a gas while the volume is kept fixed.
At low pressures, the gas behaves almost like an ideal gas, so its readings are highly accurate and reproducible.
Its temperature scale is almost independent of the type of gas used, which makes it a dependable standard.
Therefore, it is used to calibrate liquid-in-glass thermometers, thermocouples, and optical pyrometers.
Answer:B. constant volume gas thermometer