The temperature at which the thermal electric power of a thermocouple becomes zero is called
A. Inversion temperature
B. Neutral temperature.
C. Junction temperature.
D. Null temperature

Hint: The answer to this question can be found by defining the different types of temperatures derived by the thermocouple, such as the neutral temperature, the inversion temperature and the hot and cold junction temperature. These temperatures are dependent on the emf on the thermocouple.

Complete step by step answer:
A thermocouple is a device used to measure the temperature difference between any two substances. The emf of the thermocouple depends on the nature of the metal.
Neutral temperature – in a thermocouple, the temperature which is independent of the temperature of the cold junction (or temperature with maximum Thermo emf) (or temperature with zero electric power) is called the neutral temperature.
Inversion temperature – The temperature at which the thermocouple will have a zero emf is the temperature of the inversion, for a thermocouple. The Thermo emf for a thermocouple can be zero, positive or negative.
Junction temperature – The temperature at the junction of the metals in a thermocouple is called the junction temperature. The junction connected to the device for measuring the temperature is called the cold junction. The other junction with winding is called the hot junction.
If in case a thermocouple is made up of two different metals having different properties, then, when connected to a battery, one junction of the metal will become hotter than the other.

So, the correct answer is “Option B”.

Note: The thermocouple is based on the principle of the Seebeck effect. For a thermocouple, the neutral temperature is considered to be equal to $270^\circ$. The neutral temperature of a thermocouple is defined as the temperature at which the Thermo emf is maximum.