D

Understanding Thermo e.m.f. in Thermocouples:
Thermocouples generate a voltage known as thermo e.m.f. based on the Seebeck effect, which arises from the temperature difference between the two junctions made from different materials.

Step 1: Analyzing Thermo e.m.f. Values
- The thermo e.m.f. can be zero when both junctions are at the same temperature, resulting in no voltage difference.
- It can be positive if the hot junction is at a higher temperature than the cold junction, leading to a voltage output in the positive direction.
- It can also be negative if the cold junction is at a higher temperature than the hot junction, hence leading to a negative voltage output.

Step 2: Conclusion
Since the thermo e.m.f. can be zero, positive, or negative depending on the temperature difference between the junctions, the correct answer is All of the above options (A, B, C are correct). Therefore, the correct answer is D.