B

Thermocouples work on the principle of the thermoelectric effect, where a voltage is generated when there is a temperature difference between two different metals.
Step 1: Antimony and bismuth are chosen for their specific thermoelectric properties. They have a stable and constant thermal electromotive force (e.m.f.) at various temperature ranges.
Step 2: This constant thermal e.m.f. allows for more reliable measurements in thermocouple applications.
Step 3: The other options mention negative, lower, or higher e.m.f., which do not accurately describe the primary reason these materials are used.
Therefore, the correct answer is Option B: Constant thermal e.m.f. produced.