D

Step 1: Understand the concept of neutral temperature in thermocouples. The neutral temperature is significant in the context of how thermocouples operate for temperature measurement.
Step 2: Recall that a thermocouple generates a voltage (thermo e.m.f.) when there is a temperature difference between its junctions. This voltage is dependent on the types of materials used in the thermocouple and the temperature difference.
Step 3: At the neutral temperature, the thermoelectric effects essentially balance out, resulting in a situation where there is no net voltage produced (thermo e.m.f. is zero).
Step 4: Therefore, the correct understanding of the neutral temperature aligns with the condition where the thermo e.m.f. is zero.
Conclusion: Hence, the correct answer is option D.