A

Step 1: In thermocouples, the direction of current flow is determined by the materials used and their positions in the Seebeck series.
Step 2: The Seebeck effect indicates that a voltage (and thus current) is generated in a circuit made of two different metals when there is a temperature difference across the junctions.
Step 3: Given that Sb (Antimony) appears before Bi (Bismuth) in the Seebeck series, it suggests that Sb is a better electron donor compared to Bi.
Step 4: Therefore, when the thermal gradient is established, the current will flow from Sb to Bi, which is established at the hot junction if Sb is at the higher temperature.
Thus, the correct answer is that the current flows from Sb to Bi at the hot junction.
Therefore, A.